Andrew Herbert

David Hartley
email: david.hartley@tnmoc.org

EDSAC Replica Project

Keywords: EDSAC, replica, functional

EDSAC Background

The Electronic Delay Storage Automatic Calculator (EDSAC) was the world's first practical stored program electronic computer. Running its first program on 6 th May 1949 EDSAC provided a scientific computing service to Cambridge University ahead of any similar provision elsewhere. To its users EDSAC represented a thousand fold increment in computing power over the mechanical calculators that preceded it and contributed to three Cambridge Nobel Prizes by enabling faster and larger scale computations than had hitherto been practical.

EDSAC was designed by Maurice Wilkes (later Professor Sir Maurice Wilkes) of the Cambridge University Mathematical Laboratory in 1947. At the instigation of Professor Douglas Hartree, an eminent Cambridge mathematician, Wilkes attended the famous Moore School Lectures at the University of Pennsylvania in the summer of 1946 at which the American computer pioneers presented experience with machines such as the ENIAC and perhaps more importantly, Von Neumann presented a paper on the design of the EDVAC that first crystallised the principles of operation for modern stored program computers. Greatly influenced by EDVAC, Wilkes sketched a design for EDSAC while travelling home on the Queen Mary and on arrival back in Cambridge in September 1946 he assembled a team of engineers led by W. Renwick to start construction of a machine.

Events moved quickly after the first successful program ran on 6 th May 1949: the machine was completed and a conference took place the following month by which time the computer was already in regular service. EDSAC continued in use for nearly 10 years until replaced by its successor, EDSAC 2. Throughout its life EDSAC underwent continual improvement and by the end of its life, the size of the store had doubled from 512 to 1024 18 bit words, and additional instructions, an index register and magnetic tape storage facilities had been added. By the standards of the time, EDSAC ran reliably throughout its life, typically delivering 35 hours of computation a week. Outside of normal hours it often ran unattended by engineers, under the control of authorized users. This can be attributed to the conservative nature of Wilkes' design. The central clock ran at 500KHz, allowing generous margins for internal synchronisation and the architecture was serial, reducing the number of circuits to be constructed from large and expensive thermionic valves.

EDSAC was fondly remembered by its users, many of whom attended a 50 th anniversary conference at Cambridge in 1999. Sadly little of the machine survives in physical form and the documentary record in the Cambridge University library is fragmentary. Over the last decade, with the death of Wilkes and most of those who worked with him, knowledge of this important machine in the history of computing is rapidly fading away.

Project Aims

The goal of the EDSAC Replica Project is to build a functional replica of EDSAC as it was in 1949. By functional replica we are expressing the intention to build a machine that runs programs as EDSAC would have done in 1949, which appears physically as similar as possible to the appearance of EDSAC in photographs, is consistent with the surviving documentation and where we have to re-invent parts of the machine, we do so using appropriate historical circuit designs and assembly techniques. We plan to exhibit the EDSAC replica at the UK National Museum of Computing at Bletchley Park.

Our aims for the replica project are that it should:  Provide a tangible demonstration of the achievements of the Cambridge pioneers.

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Celebrate the achievements of British scientific and technological contributions to the early development of computing.

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Assemble archive material relating to this historic machine and capture the knowledge of the few remaining pioneers who developed and used it.

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Build as authentic a replica as possible taking into account the availability of components and materials and modern safety standards.

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Undertake the work within the spirit of the designers and technology of the time.

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Produce a working artefact of exceptional educational value to students and the general public.

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Be an exemplar of British engineering and encourage new students to take up computing and engineering.

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Demonstrate the working machine to the public as often as is practical once it is built and commissioned at The National Museum of Computing.

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Be constructed and equipped with sufficient spares to enable the machine to run for the next 25 years.

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Train a new generation of volunteers, unfamiliar with 1940s valve technology, to build, run, demonstrate and maintain the replica for the foreseeable future.

Project Organization

The second author of this paper (David Hartley) with the encouragement of Dr Hermann Hauser, a well-known Cambridge technology entrepreneur originally conceived the project. Led by Hartley, the University of Cambridge, the British Computer Society and the Hauser-Raspe Foundation jointly set up The EDSAC Replica Project as a registered UK charity. Initial funding came from The Hauser-Raspe Foundation, Google and a private individual who have, at the time of writing, generously contributed half of the estimated project cost of £250,000.

Volunteers are undertaking much of the construction of the replica with some work contracted out to commercial suppliers. The first author (Andrew Herbert) is the Project Manager. Equally important in the leadership of the project is the role of Chris Burton as Chief Designer. Chris brings to the project experience of the restoration of a Ferranti Pegasus computer at the London Science Museum and the construction of a functional replica of the University of Manchester Small Scale Experimental Machine ("Baby") for the Manchester Museum of Science and Industry. The target date for completion is mid to late 2015.

Wilkes was unsentimental about the machines he designed: when EDSAC 2 was ready for use, EDSAC was unceremoniously scrapped to make room for it. Just a few pieces were sent to museums: 3 examples of the most common type of EDSAC chassis and 3 short mercury delay line tubes.

A number of accessories such as hand punches for correcting paper tape also survive but that is all. Fortunately there is a reasonably good photographic record: Wilkes had official photographs made from glass-plate negatives and these remain in the Cambridge University Library. His own personal photograph album has been donated by his family to the Cambridge University Computer Laboratory to put alongside other personal photographs collected at the time of the EDSAC'99 conference. Importantly a 1951 16mm colour film made in the Mathematical Laboratory (the former name of the present Computer Laboratory) showing the operation of the machine survives. However most of the images are undated and it can be seen that significant changes were made to the machine throughout its life and therefore care has to be taken in using them to settle questions about EDSAC's appearance in 1949.

The documentary record is on the surface rather more complete. A key document is The EDSAC Report deposited by Wilkes in the University Library. This gives a good overview of the architecture of the machine and its logical design. There are hints at some of the circuit elements used but no description of the physical layout. Moreover it is unclear whether the EDSAC Report is a design specification or a write-up of the completed machine, or perhaps something in between, so it too must be used with caution.

We then have academic papers written about EDSAC. Mostly these are at the architectural level and focus on how EDSAC was programmedanother of the EDSAC innovations was the simplicity and convenience of its programming system. A few papers give more detailed engineering information, particularly about the mercury delay lines used for the main store.

Then there are Wilkes' memoirs and his speeches on EDSAC to various historical societies and computer associations, but these tend to speak more to the context in which EDSAC was conceived and used than in giving technical detail.

Finally we have some surviving notebooks from the project team and "EDSAC Operating Memoranda" and "EDSAC Programming Bulletins" issued to users of EDSAC that shed some light on technical detail. Early documents on the Lyons LEO I computer have also proved useful: the design of LEO was based on EDSAC In summary, the project has enough information to know the principles of how EDSAC worked and was used, but lacks most of the detail required to construct an exact replica. The essential project task is to bridge this gap.

EDSAC Physical Design

Physically EDSAC consisted of three rows of open racks, each row comprising four, three and five racks respectively and each rack holding up to 14 shelves containing electronics. (Wilkes called the shelves "panels", and in the project we follow more modern terminology and call them "chassis"; our mechanical engineering subcontractor calls them "shelves"). The total footprint of the racks is about 5m 2 and they are approximately 2.5m high. In total there are 142 chassis.

In addition to the racks, further floor space was taken up by wooden "coffins" containing the mercury delay line tanks used for the main store and registers. The coffins provided a level of temperature stability and were approximately 2m long. The store was later moved to a thermostatically controlled oven.

The operator controlled the machine from a simple panel with a Reset and Start button mounted on one of the racks: a paper tape reader for input and teleprinter for output were placed on a wooden table. A separate engineer's desk accommodated a three-oscilloscope unit that allowed the contents of selected memory tubes to be inspected. (Later in the life of EDSAC all these elements moved to a single operator's desk).

EDSAC was powered from a motor generator unit in a separate room: the electrical consumption is estimated at 15KW.

In terms of building a replica, the racks and chassis are relatively straightforward to reconstruct as we have a sample chassis and several photographs from which we can estimate leading dimensions. Working with a local engineering company in Cambridge (Teversham Engineering Ltd) we have CAD models for chassis and racks. In contrast to the original, where making a chassis would probably have taken an engineer most of a day in the Mathematical Laboratory's workshops, using their Computer Aided manufacturing systems, Teversham Engineering can cut, punch, drill and fold chassis in a matter of minutes.

The chassis model is a generic template as different chassis had different punching depending upon the circuits it contained. This is where difficulties arisewe do not have a photograph of every chassis and over the lifetime of EDSAC some chassis were moved, or changed and new functions introduced. To further frustrate us, we know each chassis had an identifying engraved label, and while we can see these on the We therefore have had to co-design the physical structure of the machine with the logical design and the circuit design. From logical design we can predict which circuit elements might have been used, and from those likely physical layouts of valves and test points on chassis: these can then be matched to photographs. We can further reasonably assume that chassis implementing related parts of the same set of logical functions (e.g., arithmetic, computer control, store addressing etc.) are grouped close together and from that make a well-informed guess as to where those chassis might have been placed in the racks if not fully visible.

In making the physical racks and chassis we have followed original dimensions but for convenience substituted modern metric fixings for the frames. On the chassis we have retained the BA fixings used on the original.

Surprisingly while the valves we need have been relatively easy to obtain from dealers, valve holders have been more difficult and top clips impossible. It is possible to get valve holders made in quantity but they are expensive: they can also be imported from the Far East but the quality is unsatisfactory. In the case of International Octal sockets we have located slightly oversize items and turned them down to standard size to fit the chassis.

Similarly we have been able to get tag strips for connecting components manufactured in bulk, a great convenience as we need at least two for every chassis. Customised tag strips are required for the connections between chassis: these are being hand made as each chassis type has a unique design. Fortunately a suitable punch, tags and blank paxolin strips could be obtained from the same source as the manufactured strips.

Another area requiring custom manufacture has been the provision of electromagnetic delay line inductances. These consist of wound coils on a Tufnol former wired to capacitors on a thick wire frame. Through the British Vintage Wireless Society we have been able to get the coils wound to our specification. The manufacture of formers and wiring up is being undertaken by project volunteers.

An area in which we have chosen to compromise is with passive components such as resistors and capacitors. Modern components are generally much smaller than their equivalents from the 1940s and painted differently. While old components can be found, they have often drifted away from their nominal value and the potential for them failing in use is high. We have therefore decided to use modern components for reasons of reliability. Where possible we look for modern components of a suitably large size (for example by choosing resistors of a higher power rating than necessary) so that the visual appearance is suitable. Further, through our relationship with the British Vintage Wireless Society we have located some suppliers of modern components to old designs. Given the scale of EDSAC we have decided against following some of the society's practices for hand building lookalike versions of old components. To a significant degree this compromise will not impact the appearance of the replica as most of the passive components lie underneath the chassis, and the valves, for which we are using genuine parts, are the most visible and characteristic aspect. We may choose to build one or two representative chassis with correct vintage components for illustrative purposes and to emphasise that the use of more modern ones elsewhere has not compromised the integrity of the replica machine.

EDSAC Logical Design

Recovery of the logical design of EDSAC was a key first achievement of the project. Fortunately, from the EDSAC Report and early papers about EDSAC there is a wealth of detail about the functional units within the machine and the data paths between them. The EDSAC Report also contains a number of timing diagrams showing how signals are delayed as they pass through parts of the machine. From this information Bill Purvis has constructed an EDSAC logic simulator called ELSIE that reads in a description of the machine and simulates execution of a program. By stepwise refinement Bill was able to arrive at a design that correctly executes EDSAC programs and was consistent with the functions for which we have descriptions. A number of areas such as paper tape input, teleprinter output and initial instruction loading have been left as black-box subsystems, but most of the rest of the machine has been analysed to a sufficient level of detail to allow circuit design to start.

In early versions of ELSIE signals were represented as rectangular pulses in units of clock pulse intervals. As understanding of the logic developed and circuit elements were designed, the signal model was refined to better represent actual pulse shapes and delays to help the circuit designers determine where pulse resynchronisation and reshaping was required. In the same vein considerations of the timing of signal propagation have helped decide how close the various logical units might have been to one another.

EDSAC Electronic Design

Electronic design was the area where we had least to start with: the EDSAC Report shows a circuit for a flip-flop, but without any component values stated: we had some hand-drawn diagrams for the clock and digit pulse generating subsystem and just one original chassis from the storage regeneration sub-system. From various papers and notebooks we had some references to problems with specific circuits and the names of circuit elements used. We also know Wilkes had a wartime background in radar and we can look to period textbooks to understand typical circuit elements from that context that might have been used or adapted for EDSAC. From these sketchy foundations Chris Burton has established a library of standard circuits for logic gates, pulse amplifiers and flip-flops for other project volunteers to use as they design complete circuits for logical functions such as the central clocking, arithmetic unit and so forth. Where we know the physical location of the function in the original machine and have a suitable photograph we can check the physical valve layout against the electronic design, and if necessary revise the electronic design to match the physical layout. Circuit design has been further complicated by the fact EDSAC is a serial machine and A.C. coupled making circuit principles unfamiliar to those who have grown up in the transistor age with parallel data paths and D.C. coupled circuits.

From the surviving original chassis and a few close-up photographs of chassis from the early days, Chris has also been able to lay down constructional principles for how individual components should be wired together, standard values chosen for components such as stopper resistors and so forth.

It is sobering to modern eyes to realise that a simple AND gate for EDSAC takes 3 pentode and two double diode thermionic valves, so with a maximum capacity on a shelf of about 18-20 valves, each chassis is limited to 3-4 gates only. With a total of 3,000 valves therefore EDSAC is equivalent to less than 600 gates (and indeed significantly less than this given the need for valve amplifiers to drive output signals etc.). It is easy to appreciate why Wilkes and his team went for a serial rather than parallel architecture.

A recurrent challenge in the on-going work to design replica EDSAC circuits is to avoid the temptation to design a better EDSAC. The temptation comes from several sources. We know from surviving notebooks that some parts of EDSAC were temperamental: the pioneers found workarounds for such problems rather than wholesale redesign as their driving purpose was to get to a fully working machine as soon as possible. Some circuits were later replaced by improved designs. but these are outside the scope of our goal to replicate EDSAC as it was in 1949. For example we have access to more modern components, e.g., plugs and sockets for interconnecting chassis rather than soldering wires between tag strips that would be more convenient and safer. We have been ruthless to avoid such temptations with two exceptions. First we are putting a 0V (zero volt) rail on each chassis to help with electrical safety: on the original 0V rail was the chassis itself and chassis were tied together and thence to ground. This is a reversible "improvement"we can chose to connect each such rail to chassis if we change our minds. Second, while we continue with the principle of using wiring between tag strips to interconnect chassis, we are using modern grip fasteners ("fastons") that clip tightly to the tags. A related concern is the distribution of power and high voltage DC around the machine. The original machine had a motor generator delivering power to each rack that was distributed by soldering wires between short tag strips. We are exploring the possibility of using smaller modern power supplies on a per rack basis and modern connectors to feed power to each chassis for the obvious safety reasons. We also plan to use modern RCCBs for earth leakage and electric shock prevention. In these respects we will be thoroughly in the modern world, and in that spirit our working practices when commissioning and operating the machine will be subject to appropriate levels of health and safety managementsomething clearly not on the minds of the pioneers judging by some of the hazards visible in the early photographs!

For archival purposes, all of our circuit designs have been captured digitally using "DesignSpark", a circuit design and PCB layout program available from one of the major suppliers of electronic components (RSComponents). We also have a formal system of "Hardware Notes" for capturing key design decisions and principles and investigations into prototype circuits etc.

EDSAC Memory

The main store of EDSAC and its internal registers were constructed using mercury delay lines. This was a technology familiar to Wilkes from his wartime radar work, and through a colleague, Tommy Gold, he had the expertise needed to make mercury delay lines for EDSAC. These delay lines present a significant challenge to the replica project. Mercury is expensive and has to be handled as a dangerous substance. The precision engineering required to manufacture the required tubes and end plates is demanding. In service, the delay lines were temperature sensitive and a source of unreliability. Moreover the mercury attacked the seals in the units that had to be replaced and the mercury itself "cleaned" every couple of years.

All these considerations make replicating a mercury delay line store impractical. During the commissioning stage we plan to build an interim store using a modern microcontroller so that, in addition to simulating delay lines, we can provide facilities to preload and inspect store. We will arrange that the interim store has the same electric signal input and output as the original mercury delay lines so that the store access and regeneration circuits will be as in the original, and the option to build a mercury delay line store in the future remains open. One project member, Peter Linington, has been investigating "nickel delay lines" as an alternative to mercury delay lines. Nickel delay lines were a popular storage technology in the 1950s, bridging the gap between mercury delay lines and ferrite core store. The current plan is to use these to replace the interim microcontroller store. They are a close physical analogue to mercury delay lines in that both use a mechanical transducer to send an acoustic pulse through a medium to "store" it. Conveniently they are also of similar dimensions. While there are surviving example nickel delay lines that can be inspected and technical papers on the physics of the medium, the means of constructing them has essentially been lost. Peter has been investigating techniques for annealing the wires, welding the fingers that actuate them and suitable materials to support the wires and to dampen out echoes etc. At the time of writing, he has a sample short delay line, suitable for uses as a register, which operates reliably with an error rate of 1 in 10 11 , i.e., roughly one bit error per day of operation. Current developments are focussed on building a long delay line suitable for use in the main store. To fit an EDSAC store "coffin" the wire needs to be coiled or folded in half. Techniques to do this are under investigation.

As a long-term ambition we are giving consideration to replicating a short delay line tank (or possibly re-commissioning one of the surviving originals) as a standalone demonstration of how mercury delay lines work.

EDSAC Input-Output

The original EDSAC took input from 5-hole paper tape, read by an electro-mechanical reader constructed in the Mathematical Laboratory.

From Wilkes' personal photographs we have good views of the internal mechanism and John Deane has been sketching a design for manufacturing a replica. We have from the original the sprocket wheel required to pull the tape through. Holes in the tape were mechanically sensed and relays used to capture each row of data and signal it electrically to the computer.

Output was to a teleprinter and again we have some photographs of the original. Bill Purvis is investigating how a Creed teleprinter from the 1950s could be adapted to meet our needs. Two related items of "input-output" are the means for loading the initial orders when the Reset button is pressed and the "3-oscilloscope unit" on the engineer's desk. We know from the documentation that the initial orders were wired on a bank of telephony uniselectors and injected into the main store. We also have a photograph of the unit in the original machine. Fortunately uniselectors are still obtainable, and Andrew Brown is in the early stages of designing the initial orders loading subsystem. The 3-oscilloscope unit allowed the operator to select a bank of words in the store and see a visible representation of their content on one of the scopes and the current instruction and its address on the other two: with an execution speed of around 650 instructions per second it was relatively easy to monitor program execution in this way. Currently little work has been put into understanding how this unit was connected or designed internally, but is clearly a critical part of the original that needs to be re-created.

For the initial commissioning phase of the project, and as an operating convenience thereafter, we plan to use a modern PC as a substitute for tape reader and teleprinter (and as mentioned earlier for store also). The details are yet to be worked out but our aim would be to achieve plug compatibility at the lowest level so we can easily switch out the PC and run with real peripherals.

By setting the goal that the replica match EDSAC as it was in 1949, the project team set a challenge to those interested in programming the machine. The first programs ran using "initial orders 1" but in September of that year these had been replaced by David Wheeler's "initial orders 2" which introduced facilities for subroutine linkage and relocation, making it practical to write large programs using a library of standard functions. To run the iconic first programs (table of squares, list of prime numbers) we need initial orders 1. We knew that Wheeler had written a socalled set of "coordinating orders" that allowed initial orders 2 programs to be loaded under initial orders 1 with the coordinating orders as a prelude on their tapes. Unfortunately no description of the coordinating orders survives, and knowing of Wheeler's reputation as a masterful machine code programmer the software team doubted these could be replicated. Somewhat as an experiment we tried using initial orders 1 to load initial orders 2 to the top of store and then copy them down to the bottom end. Much to our delight this worked and so the secret of Wheeler's coordinating orders has been successfully penetrated!

Current Status

At the time of writing volunteers are wiring up chassis as follows:

1. The central Clock Pulse Generator -Peter Lawrence 2. The bank of Digit Pulse Generators -John Sanderson 3. The half adder (at the core of the "arithmetic unit") -Nigel Bennée 4. The storage access logic (tank address decoding) -John Pratt.

We have a prototype nickel delay line short tank for register memory -Peter Linington.

Figure 1 :

 1 Figure 1: EDSAC, May 1949

Figure 2 :

 2 Figure 2: Surviving EDSAC Chassis

Figure 3 :

 3 Figure 3: Chassis Underside

Figure 4 :

 4 Figure 4: Illegible Chassis Label

Figure 6 :

 6 Figure 6: Typical ELSIE Output

Figure 7 :

 7 Figure 7: EDSAC Digit Pulse Generator Chassis

Figure 8 :

 8 Figure 8: Wilkes and Mercury Delay Lines

Figure 10 :Figure 9 :

 109 Figure 10: EDSAC tape reader and teleprinter

Design is underway for the tape reader -John Deane, teleprinter -Bill Purvis and initial instruction loading -Andrew Brown.

In addition to the named designers above working with Chris Burton our Chief Designer, we have a further 6 or so volunteers helping with construction and most importantly, Alan Clarke who is our storekeeper and seeks out sources for all the parts required.

The aim is to have a first fully operating version of the replica at TNMOC with interim store and I/O by late summer 2015. As a first working exhibit we hope to have a rack on display with clock pulses and counting (i.e., adding) on display this summer (2013).

Summary

The goal of the EDSAC Replica Project is to build a functional replica of The University of Cambridge EDSAC Computer as it was when first operating in 1949 and for the replica to be a working demonstration at the UK National Museum of Computing. Little of the original EDSAC survives, but working from photographs and a fragmented documentary record, the project team have resolved many questions of physical, logical and electronic design and are, at the time of writing, constructing the first set of functional EDSAC chassis. The replica will use nickel delay lines for the main store in place of the mercury tanks of the original, but in all other respects the replica will be as close as possible to the original. The target date for project completion is 2015.


Kevin Murrell
email: kevin.murrell@tnmoc.org

The Harwell Dekatron Computer

Keywords: Harwell Dekatron Computer, restoration, WITCH, National Museum of Computing, Dekatrons

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Introduction: Restoring the Harwell Dekatron Computer

Design of the computer began in 1949 using techniques that were generally well understood, and used components that were available to the Harwell engineers at the time. There seem to have been remarkably few problems with the design when it came to commissioning the machine and running the first few tests, and the computer was completed and first made to work properly in April 1951 using just two of the store units. This was sufficient to prove the machine was functioning correctly, and to allow simple programs to be written to test the machine more fully. By 1952, the team had added the remaining Dekatron store units and handed the computer over to the theoretical physics division for proper use.

The very conservative design of the computer was quite deliberate as it led to a reliable workhorse that could be left unattended to plough through a long series of calculations. Harwell kept precise records of the work done by the computer, and in a paper published in 1953 it was reported that in its first year with the division, from May 1952 to February 1953, the machine ran on average 92½ hours each week or for 55% of the total available time. Typically any lost time was when the machine had finished its allotted tasks and was waiting for its operators to arrive the next morning! By 1957, the mathematicians at Harwell were making little use of the Dekatron Computer, but did not wish to see the machine broken up, so an imaginative competition was devised to make further use of the computer and give it a new home in education.

On the 25 th February 1957, Wolverhampton and Staffordshire College of Technology announced to the world they had won the competition and would soon receive the computer from Harwell. The official commissioning day in Wolverhampton was Wednesday, 4 th December 1957, and at this point the machine was re-christened 'WITCH' -the 'Wolverhampton Instrument for Teaching Computation from Harwell'.

The college quickly established both a full time degree and a Diploma in Technology, and ran regular courses, many in the evening, to introduce computing to local colleges and schools. Computing moves at a frenetic space, and this was especially so during the 1960s and 1970s. The college made full use of the computer until 1973 when it was formally switched off and given to a local museum.

The computer was displayed in the museum for some time, but was eventually dismantled and put into long-term storage. In 2008, the author, by then the secretary of the Computer Conservation Society and a founder trustee of The National Museum of Computing in the UK, discovered a few parts of the machine and set about tracking down the remains of the computer. Quite quickly we discovered the bulk of the computer, albeit broken down into many pieces.

By now, there was a feeling of genuine excitement that this machine could be restored to working order and presented again for the public to see and appreciate. The work on collecting information about the machine continued in earnest, and again fate intervened when at an unrelated BCS (British Computer Society) meeting, we were given the contact details for Ted Cooke-Yarborough who was living near Abingdonnot far from Harwell. Ted was able to contact Dick Barnes, and very soon a meeting was arranged to tell them of our re-discovery of the machine and our plans for its restoration. The project team travelled to Abingdon in March 2009 to meet the two engineers to show them the results of our investigations and discuss our hopes to restore the machine to full working order. Barnes had kept a scrapbook of newspaper cuttings that proved very useful in understanding the history of the machine.

In April 2009 the project team wrote a formal restoration project plan to define what we hoped to achieve, what we needed to complete the project, and to detail any risks along the way. This plan was presented to the Computer Conservation Society (CCS) in May 2009 and was accepted as a new CCS Working Party. The CCS also kindly offered funds towards the initial costs of the project.

Some of the considerations we discussed were:  Might we simply re-assemble the machine back to its original condition?  Is enough of the original system available to consider its restoration to working order?  Do we have the necessary tools and documentation?  Do we have the sufficient spare parts that might be needed?  How do we decide what might be replaced?  Is original software available or accessible?  Do we have the funds and the space to do this?

In September 2009, the machine was moved to its new home -the Large Systems Gallery at the National Museum of Computing. Even before any restoration work commenced, it was already attracting the curiosity of visitors, many of whom had never seen a computer looking anything like this machine with its rows of Dekatrons, valves and trigger tubes, and mysterious grey canisters filled with relays.

Our objective therefore was to restore the machine to a state where it could be reliably and safely operated for demonstration to the public and the many educational and corporate groups who visit the museum.

In common with all our restoration work, the philosophy is to change as little as possible, particularly external appearance. Although it may be tempting to re-spray a rusty old machine, as one might routinely do when restoring a classic car, we feel this alters the character of the machinewe want our historic exhibits to look their age! In general we confine our activities to cleaning and conservation to prevent further degradation. On the Harwell machine we used electrolysis to treat the areas worst affected by rust.

This computer has had a uniquely long working life, during which it was extensively modified and repaired. We decided early on that we would not attempt to return it to an 'original' condition, and indeed, as work progressed, it became clear that establishing the exact details of the original design would be rather difficult in some areas.

Particularly in the power supplies, it was necessary to replace failed elderly components that are no longer available, some of these being quite prominently visible. In those cases the original components have been left in place, with the much smaller modern replacement components hidden from view. This helps to ensure the machine retains its unique character.

Restoration work fell into three main categories: relays, mechanical, and electronics. The machine came with full circuit diagrams, and an excellent detailed technical description written by Dick Barnes at Harwell. There was also a reasonable quantity of spare components.

Once work had begun in earnest we quickly discovered that the machine deviated from the diagrams in a number of areas, and furthermore that modifications had been made for which documentation hasn't survived. This has made the work more challenging than it might otherwise have been, and efforts are still on-going to fully document the machine in its current state.

Relays

The machine was built using Post Office 3000 type relays, which were at the time a mature and well understood technology. They were designed to operate for decades in the harsh environment of a telephone exchange, and consequently were extremely well made. A number of relays were discovered with bent contacts, and one concern was they may have been deliberately set out of normal adjustment to compensate for some timing problem within the design of the machine. In the event we have discovered very few areas that depend critically on relative relay timings.

The long and tedious job of relay adjustment has been rewarded by relatively trouble free operation of the complex relay logic of the machine, which at the time of writing has been working for over two years. During this period the machine has been regularly demonstrated to museum visitors by means of a simple program that reads from two tape readers alternately using block search and transfer control instructions. We anticipate an on-going need for periodic cleaning and adjustment of the relay sets.

Mechanical

The output equipment consists of a Creed 7B printer, a Creed 75 tele-printer and a Creed 7TR re-perforator. All of these were specially modified at Harwell to operate with this machine. The printer and re-perforator date from the early 1940s, and so predate the rest of the machine by quite a few years.

The standard Creed models used serial I/Oa more primitive version of the RS-232 serial port which until recently could be found on every modern PC. By contrast this machine uses parallel 5-wire I/O. There are standard 5-bit character codes, but in common with many other early computers this machine used a custom character set. One reason for this was to simplify the logic required to assure correct operationthus the numbers 0-9 that form the instruction code are all represented by combinations of two holes, and the block marking codes used to delineate program or data sections by combinations of three holes. This meant relatively simple logic could detect when an unexpected code was encountered and halt operation.

Restoration of these machines involved nothing more than a good clean and some adjustmentsthey really were built to last! Again we are fortunate to have experts in this technology at the museum who were able to carry out the work.

Electronics

Sequence control is entirely relay based, but arithmetic is all electronic, built using valves, trigger tubes and Dekatrons. There are also two high voltage power supply unitsthe rectifier unit converts the AC mains into several high DC voltages, and the stabiliser unit generates about a dozen precise DC voltages for use in the various circuits.

The rectifier unit has been extensively modified during the operational life of the machine, and now sports a variety of rectification technologies dating from the 1950s (copper oxide) to the present day (silicon diodes). We had to replace some of the original copper oxide rectifiers with modern components, but others had already been replaced during the machine's time at Wolverhampton. The other major problem in this unit was a leaking oil-filled transformer, eventually solved by copious use of Araldite. All the transformers in the machine were tested to ensure the insulation was still in good condition.

Once the power supplies were operating reliably we turned our attention to the pulse generator, which is one of the most complex parts of the machine. Its main job is to generate various sequences of pulses to step the Dekatrons, but it also has a hand in starting arithmetic operations, deciding when they are complete, and controlling the transfer and carry units which perform the actual addition and subtraction operations. Some of the subtler (and undocumented) aspects of its operation only became clear when it was back on the racks and interacting with the rest of the machine.

The majority of the work was done with the pulse generator on the bench, with a few switches connected up to provide control inputs. We found around half a dozen failed componentsmainly diodes and capacitors. The other major issue has proven to be the high-speed trigger tubes which start and stop the Dekatron used in the pulse 306 generator to count out ten pulses. We had a large supply of spare tubes that came with the machine, but the majority turned out not to work. We were lucky to find a supplier who allowed us to test their entire stock of several hundred, but this only yielded a small number of working ones, and some of these no longer work at the time of writing. The most likely explanation is the gas has escaped around where the pins enter the basethis is surprising since the problem of making valves reliably gas tight had been largely solved by the early 1950s. Keeping the machine running long term will eventually require a modification of these circuits to use a more reliable type of trigger tube.

The remainder of the units in the arithmetic rack were repaired one at a time on the bench with the help of the pulse generator and power supplies. These consist of the two halves of the accumulator, which also contain plug-in transfer and carry modules, and the translator unit that converts between 5-wire code and Dekatron compatible pulse trains. The translator is used both when reading numbers from tape into store and when printing out from store. No major issues were found with any of these units bar a handful of failed components.

With the arithmetic rack completed we turned our attention to the stores. The only major problem that Dekatrons suffer from is contamination of adjacent electrodes due to the glow being left on one cathode for a long period of time. This results in 'sticky' operation -either the glow won't reliably step past a particular cathode, or jumps straight over it. Both of these problems cause an incorrect answer. Luckily the process can be reversed by simply allowing the Dekatron to 'spin' for a long enough period.

No detailed operational log of the machine survives, but it's likely some of these faults have been present for many years and were probably partly responsible for the unreliability which we know the machine developed in its later life. These circuits must only just have worked when the machine was new, and hence even small component value changes caused a problem.

The goal of the restoration was to have the machine fully working and as reliable as it was when new. At the time of writing this has largely been achieved, with only a few reliability issues left to solve. The machine is once again able to run the original programs written for it in the 1950s.

The museum plans to produce teachers and students information packs about the machine that can be downloaded in advance of their visit. The packs will include detailed instructions on how the machine is programmed, example programs and potential programming challenges. We also hope to include a software emulation of the computer so that students can prepare programs prior to their visit, and then on the day actually run them on the live machine.

Very little remains of the history of computing in the UK from the late 1940s and early 1950s, and we are very lucky that this machine has led such as charmed life and managed to 'avoid the chop' on several occasions. Its important early role in pioneering nuclear technology, its long second life in education, and its third life as a museum exhibit enthusing young people, can now continue as an active machine enthusing and helping train a new generation of engineers and programmers.


Benjamin Trethowan
email: ben.trethowan@tnmoc.org

Capturing, Restoring, and Presenting, the Independent Radar Investigation System (IRIS)

Keywords: ANSP, ASTERIX, ATS, BEG, CAA, CCC, DEC, DEG, IRIS, LATCC, NATS, NERC, PDP, PRDS, RARE, RBEG, RDI, RDP, TNMoC

This paper describes a recent project at The National Museum of Computing (TNMoC) to capture, restore to working order, and present to the public, an early air traffic control system. It discusses the importance of capturing an extensive range of information, relating to the system, at the point of donation, the value of this information within the restoration process, the techniques used within the restoration itself, and the value of expressing the social impact of the system in order to convey its relevance to the public.

Introduction

The Independent Radar Investigation System (IRIS) is an isolated Radar Data Processing (RDP) system, originally used for the investigation of air traffic incidents. It allowed recorded radar and voice (i.e. radiotelephony) data to be replayed together, in synchronisation, for investigative purposes. It was designed and built in the early 1970s at the London Area and Terminal Control Centre (LATCC) in West Drayton, Middlesex, on behalf of National Air Traffic Services (NATS); the UK's Air Navigation Service Provider (ANSP) for all en-route, and a large proportion of terminal, Air Traffic Services (ATS).

The IRIS was in use until the LATCC facility was finally decommissioned in January 2008, when air traffic operations transferred to the New En-Route Centre (NERC) facility in Swanwick, Hampshire. During its operational lifetime, the LATCC facility handled over 48 million aircraft movements.

Prior to the decommissioning of the LATCC facility, NATS had approached The National Museum of Computing (TNMoC) in order to donate the IRIS. It was felt that the IRIS represented a key element of the history of the UK's transport infrastructure, as well as an excellent example of early British computer engineering. TNMoC accepted the IRIS into its collection in April 2008, following its disassembly; packaging, and transport from the LATCC site in March 2008. Both organisations were keen that TNMoC should restore the IRIS to full working order, such that the system could be demonstrated for the enjoyment of the visiting public.

Between May and September 2008, a small team of volunteers at TNMoC carefully reassembled and restored the IRIS. Restoration activity was completed in October 2008, when the IRIS completed its first successful playback after departing the LATCC facility.

Background

The IRIS system is itself a small, standalone installation of a much larger RDP system, the Processed Radar Display Subsystem (PRDS). The operational requirement of PRDS was to "accept radar plot and label data, flight plan data, etc. and to process and display this data." [START_REF]Processed Radar Display Sub-system for the Civil Aviation Authority[END_REF] In short, the PRDS provided air traffic controllers with their operational situation display, indicating the current position of all aircraft within the selected radar range and system coverage.

The PRDS was not the primary means of radar data processing, but provided a bypass function "intended to provide an alternative radar display service when the Central Computer Complex (CCC) is unavailable." [START_REF]Processed Radar Display Sub-system for the Civil Aviation Authority[END_REF] In short, the PRDS accepted processed radar data, from a primary source, and displayed this to an air traffic controller during normal operations. However it also provided an alternative means of processing radar data, known as 'bypass', should the primary source fail for whatever reason.

The IRIS is composed of a small subset of the PRDS subsystems, representing the smallest / simplest collection of subsystems necessary to generate an operational situation display from recorded radar data. The IRIS is capable of processing a maximum of three radar data channels simultaneously, and displaying a maximum of two of these channels, on an operational situation display, at any one time.

The PRDS system architecture, and hence the IRIS system architecture, is based upon the use of the Digital Equipment Corporation (DEC) Programmed Data Processor (PDP) -11 range. The IRIS uses four such PDP-11 processors, as follows; two PDP-11/34 processors for the Display Equipment Groups (DEGs), used to render vector graphics onto two operational situation displays, one PDP-11/84 processor for the Bypass Equipment Group (BEG), used to process two radar data channels, and one PDP-11/84 processor for the Redundant Bypass Equipment Group (RBEG), used to process one radar data channel and control / monitor all other systems.

To achieve playback within the IRIS; recorded radar data is fed into the RBEG and BEG systems for processing, the processed radar data is then passed onto the DEG systems for rendering, with the vector graphics commands then being passed onto the operational situation display (also known as the Sector Equipment Group (SEG)) for drawing onto the screen.

Both the RBEG and BEG systems continually process incoming radar data, regardless of the channel; range, or other selection made at the operational situation display. The DEG systems react to input from the operational situation displays, and are responsible for rendering the display in accordance with the selected channel; dual-channel, range, map overlay, and other user-selectable settings.

Capture

At the time of the original donation, both NATS and TNMoC were keen to ensure that as much supporting material / additional artefacts, relating to the IRIS, were captured as soon as possible. The LATCC site was due to be demolished shortly after the facility was vacated, so the only opportunity to accept and preserve this supporting material would have to be taken at the earliest possible stage. The exploitation of this opportunity, and the broadness of the material captured, later proved to be a vital element in achieving the successful restoration of the IRIS.

One of the first issues experienced during the restoration, which was only resolved as a result of the original broad capture, was that of bespoke system design. Despite the fact that the IRIS subsystems utilised a popular computing platform of the period (DEC PDP-11), it transpired that a large amount of the hardware within these platforms was either of a bespoke design, or had been modified. One such example concerned the Radar Data Interface (RDI) hardware contained within the RBEG and BEG systems. Whilst the RDI hardware was modeled on a familiar full-size UNIBUS card, for obvious reasons, the microelectronics upon the card were developed specifically for the PRDS by the systems' original supplier, Plessey. Initial problems with the RDI hardware, relating to component failure, were only resolved during the restoration process due to the availability of the original Plessey schematics, which had been captured alongside the IRIS at the point of donation. Had this information not been captured when it was, it would have long since been lost; given that the demolition of the LATCC site would have progressed several months by this stage. Considering also that the PRDS implementation was unique, there would have been no alternative source for this information.

A similar issue experienced during the restoration, resolved again by the availability of supporting material, was that of legacy proprietary data communication protocols. The UK Civil Aviation Authority (CAA) had developed a proprietary protocol for the exchange of radar data, based upon a synchronous serial method of communication. The IRIS subsystems were designed to accept radar data only in this format, and were not able to process more recent radar data formats such as the All Purpose Structured Eurocontrol Surveillance Information Exchange (ASTERIX) format. The need to monitor, test, and generate, radar data for the IRIS RDP subsystems, spawned a further need to first understand the CAA radar data format itself. Luckily, the original specification for this radar data format, issued by the CAA, had also been captured alongside IRIS at the point of donation.

In addition to the engineering issues mentioned previously, the successful restoration was also dependent upon gaining an acceptable appreciation of the complex operational context within which the IRIS system originally operated. The PRDS subsystems, from which IRIS is built, originally interfaced with a multitude of additional operational systems, performing functions such as aeronautical messaging; code / callsign conversion, meteorological data communication, and flight plan processing, etc. It was therefore necessary to understand how the IRIS subsystems should operate without the support of these additional systems, and how that might affect the behaviour that the IRIS would exhibit in its initial stages of operation, following restoration. Again, the initial capture had managed to preserve complete PRDS schematics, including training course material on how the PRDS operated with the additional LATCC systems. Without the availability of this sort of documentation, the successful restoration of the IRIS almost certainly would not have been possible.

Restoration

As well as reaping the benefits of a broad capture at the point of donation, the restoration team also utilised a number of other techniques to combat additional challenges and ensure success.

Unfortunately, it is almost never possible to capture absolutely everything that might be required to facilitate a successful restoration. Setting aside the common issues of storage capacity, transport costs, and underfunding, it is often the case that some supporting artefacts have already been lost; prior to the initial offer of donation being made.

For example, the technical documentation associated with a radar data recording and replay system, supplied alongside the IRIS, had already been lost; prior to the removal of the IRIS and its associated artefacts in April 2008. This recording and replay system, known as RARE, would later prove to be an essential element in achieving the successful restoration of the IRIS, by acting as a source of the CAA format radar data necessary to achieve playback. A number of issues were initially encountered during the restoration of the RARE system, many associated with the total lack of documentation and / or understanding of how the system should respond in certain situations, or when given certain stimuli. In an attempt to combat these issues, the restoration team contacted the original donor, NATS; made a number of external enquiries, and conducted a small amount of research, in order to trace a technical contact for the RARE system that might be able to provide the necessary information. After a number of weeks, an individual who had not only worked for the original manufacturer of the RARE system, but who had also been personally involved in its design, contacted TNMoC to offer his assistance. The information that this individual was able to provide, either in hard copy form, or through technical interchange meetings, led to the successful restoration of the RARE system, and subsequently the IRIS.

A similar situation was encountered in relation to the IRIS subsystems themselves. In this case, the issue was not that the supporting material had not been captured, or lost, but that it simply did not exist. A recurring issue, observed within the power supply units of some of the IRIS subsystems, did not appear to have been documented within any of the material available to the restoration team. After extensive investigations, the restoration team contacted the original donor, NATS, in order to ascertain whether any similar issues had been observed whilst the system was in service at the LATCC facility. Following discussions with NATS, it transpired that the issue we were observing was, in fact, a known one; and was the result of incorrect technical documentation. Whilst many of the existing NATS engineers were aware of this issue, and its cause, this information was not passed on to TNMoC at the point of donation as it had, quite simply, been forgotten about. The support of the original donor, and existing third-party suppliers, proved to be vital in these circumstances.

Challenges

Whilst many of the challenges encountered during the restoration process were resolved through the use of supporting material, or through contact with the original donor and its third-party suppliers, some could only be resolved through the successful application of engineering best practice.

One such challenge was that of degraded magnetic tape media, a common feature of many computer restoration projects. An additional radar data recording and replay system, known as the SE7000, had been supplied alongside the IRIS and accompanied by a number of radar data recordings on multi-track magnetic tape. The SE7000 is, quite simply, a multi-track magnetic tape deck, with multiple read / write heads that are used to record / replay multiple radar channels simultaneously. When inspecting the magnetic media that we had been given, it became apparent that the media was suffering from the common problem of oxide separation. The restoration team was fortunate enough to have been given some additional media however, which did not exhibit the separation issue. Thankfully, this has meant that we have been able to stop short of attempting to restore the degraded media, using a potentially hazardous process, for the time being.

Similar degradation issues were found to have affected the drive mechanism itself, with numerous rubber capstans and drive belts, etc. having perished. Contemporary replacement parts were found to resolve these issues however, allowing us to operate the SE7000 unit temporarily, in order to capture the radar data from any operable media onto a more suitable modern format.

Interestingly, the more recent RARE system suffered from similar issues, despite its use of closed tape cartridges (Exabyte) instead of open tape reels. A series of recurring issues were encountered with the Exabyte drive mechanisms, which had not been maintained since their withdrawal from service many years before the point of donation. A lack of suitable spares further exacerbated this problem, and led to the decision to immediately retire the RARE unit; following the successful capture of radar data from any operable media onto a more suitable modern format.

Social Impact

After defeating the many challenges of restoration, and successfully demonstrating the IRIS in full working order since October 2008, it is perhaps worth considering some of the motivations behind this project.

The IRIS offers us a unique opportunity to convey the impact that computing technology has had upon society, demonstrating why the history and development of computing is relevant to our visiting public. Our visitors have the opportunity to both study and interact with something that was a vital component of the UK's transport infrastructure for over 25 years, something that they can immediately identify with if they have ever flown away on holiday, or to a business meeting, etc.

It also delivers a significant visual impact as an operational exhibit; its size, and 'retro' appearance, provide a visually captivating display, far removed from the socalled 'beige boxes' that many of our visitors expect to see.

And finally, it presents us with an additional educational opportunity to captivate the many young people that visit our museum, and explain just how the development of computing technology has kept, and continues to keep, us safe in the sky.


Tilly Blyth
email: tilly.blyth@sciencemuseum.ac.uk

Narratives in the History of Computing: Constructing the Information Age Gallery at the Science Museum

Keywords: Museums, displays, history of computing

One of the challenges of exhibiting the complex, and mostly intangible, world of computing in a museum context is how you bring together the technology with the people involved and the information shared. The history of computing is not just a neat history of devices. Analogue, digital, mini, personal and supercomputers all reflect the material culture of information and communication technologies, but the story of information machines is a much more complex story of the interrelationship between networks of people, societal and cultural influences. This paper reflects on approaches to the display of the history of computing and suggests that a shift to narrative and users, rather than chronology and technological progress, invites a more engaging experience for the majority of visitors. It also suggests that there is an inherent value in the display of computing artefacts that goes far beyond that of working machines. Some machines can work on a profound level, not just a utilitarian one.

Displaying the History of Computing

Texts on the history of computing tend to follow a standard sequential narrativefrom calculating aids to mechanical, digital, and then electronic devices -the expected approach to history favours an understanding that is technologically driven. Exhibition displays of the history of computing have also tended to reflect as technocentric, chronological approach. Timelines offer a clear way in for the visitor, providing a strong structure and a clear sense of the development of a technology.

A beautiful historical example is IBM's exhibition A Computer Perspective, which opened in 1973 at their Corporate Exhibit Center in New York [START_REF] Eames | A Computer Perspective[END_REF]. Developed by the designers Charles and Ray Eames, the exhibit explored the history of computing though key objects in the centre of the space and a "History Wall" along the back that acted as a multi-layered timeline documenting key events in the development of the computer.

Despite following a strictly linear structure that placed the technology at the heart, the "History Wall" played with ideas and people, alongside the technology, through its dense layering of labels and facts. Following an approach that can be best described as visual hypertext, the display created connections through the careful placing of imagery, text and objects, to show a network of ideas, people and events that led to the development of calculation, automation, computation and artificial intelligence.

The Science Museum's own computing gallery, Computing Then and Now, which opened just a few years later in 1975, is another interesting example of how contemporary ideas about computing were mapped onto a gallery space. The gallery had analogue representations of computing on the North side and Digital on the South. It provided a progression from early aids to calculation, to a 1930s punched card office, to the development of electronic digital machines, elements of a contemporary computer (input/outputs, storage and processors), ending in state-of-the-art minicomputers. Although thematic as well as chronological, the gallery clearly provides a sequence through which visitors can build their understanding of the development of computing.

Despite the obvious benefits for audiences in terms of clarity about where they are in their gallery experience, critics of a chronological approach argue that such displays provide an interpretation of history that presents the past as an inevitable progression towards the present. This Whig historiography leads to technological determinism, where the development of technology follows a predictable and predestined path, rather than reflecting and developing according to our social structures and cultural values. Technology is thought to have an effect on society, rather than being part of a network of actors working together to create meaning and use through technological development. Clearly, if we subscribe to a social constructivist approach [START_REF] Bijker | The Social Construction of Technological Systems: New Directions in the Sociology and History of Technology[END_REF] to the history of technology, we need to present a display with a more nuanced historical understanding, which places people and their context alongside the development of technology. We need to show how many of the routes of technological change were not pre-determined, but the result of parallel developments, or social, cultural and economic forces. This potentially shifts the display from a predictable and linear history, to view the development of computing in terms of ideas, personal motivations, or broader social and economic forces. It invites a more active relationship to our past, asking audiences to consider the social structure and knowledge frameworks in which our ancestors existed, and to consider individuals as actors in the history of technology that go far beyond the initial inventors and innovators, to those that used, developed and gave meaning to 'new' technologies .

Importantly, it also begins to bring the history of computing closer to a history of information, breaking the linear development of technology to invite visitors to see people, ideas and technology as relating and interlinking with each other. It suggests that linear and chronological displays limit the complexity of ideas we can present and limit the connections that visitors themselves may make.

Narratives in the History of Computing

In his book Beyond the Glass Case (2000) Nick Merriman suggests that people consume the past in two ways: through a personal past (a sense of the past experienced in personal terms) and through impersonal heritage (e.g. regional, national, international histories). This implies that museum curators not only have a responsibility to present the history of computing in a more dynamic and personal way because of a sense of 'good' historiography, but because of their public purpose to enthuse people in their own sense of heritage. Purely appealing to an impersonal heritage, such as the development of computing as an innovation driven story of monocausal progression, may do wonders for those who are already interested in the technology or the importance of changes in the computing industry, but it is unlikely to enthuse those who perceive themselves to have no immediate personal link to the development of computing (even though they may 'use' computers every day).

Rather than presenting the history of information as an inevitable consequence of past developments, museums can provide audiences with the tools for questioning the role and form of information technology in society. By viewing technology as a historical and cultural form, disseminated and appropriated by different users in different ways at different times, audiences are invited to orientate themselves within a broad history of information technology. In this way museums can become "enablers where people can harness their creative energies to construct a meaningful past of their own" (Merriman, 2000, p.95). Most importantly, it invites audiences to think, make leaps of imagination or comparisons with their ancestors, rather than read and learn.

So what techniques are open to museums in order to do this in an engaging way? Story telling is a common language for all of us from a young age. Through books, television and radio we have all enjoyed stories, but it is a form that has been relatively neglected in museology, particular science and technology museums. A few academics have highlighted the importance of storytelling for engaging people with science [START_REF] Linett | Interview: Ben Lillie on Science and the Storytelling Revival[END_REF] and developing enthusiasm for technical subjects such as computer science [START_REF] Impagliazzo | My Fascination with Computing History[END_REF]. The museum professional Leslie [START_REF] Bedford | Storytelling: The Real Work of Museums[END_REF] noted that 'Stories are the most fundamental way we learn. They have a beginning, a middle, and an end. They teach without preaching, encouraging both personal reflection and public discussion. Stories inspire wonder and awe; they allow a listener to imagine another time and place, to find the universal in the particular, and to feel empathy for others. They preserve individual and collective memory and speak to both the adult and the child' (p.33).

A strong story provides a great way of connecting visitors, enabling them to imagine the motivations of individuals and the constraints they were under. Each story has a linear structure, so the visitor is clear where they are in the story and how much longer they are required to focus. Importantly, if it is effective it can connect the visitor to history at an emotional level, so that they might invest more time to build a framework for understand broader historical events or connect with a scientific understanding. If the development of technology shifts from being an impersonal act that follows a predetermined route, to multiple stories that are defined by human values and beliefs, then visitors are more able to project their own thoughts, feelings and memories onto the story.

Shifting Technological History to the User

As historians and sociologists of technology and media began to question the role of the user as agents for social change in the 1980s and 90s [START_REF] Bijker | The Social Construction of Technological Systems: New Directions in the Sociology and History of Technology[END_REF][START_REF] Schwartz | The Consumption Junction: A proposal for research strategies in the sociology of technology[END_REF][START_REF] Silverstone | Information and communication technologies and the moral economy of the household[END_REF], they illustrated the need to breakdown the linear model of technological innovation and diffusion, to show that users of technology are not passive consumers, but actively involved in the creation and domestication of technology. More recently [START_REF] Oudshoorn | Introduction: How Users and Non-Users Matter[END_REF] stressed the importance of the representation of users in the co-construction of technology, whilst the historian David [START_REF] Edgerton | The Shock of the Old: Technology in Global History Since[END_REF] suggested that we should refocus the history of technology on apparently 'old' technologies that are commonly used, as the majority of our world is reliant on the technologies we utilize, rediscover and redevelop, rather than on discoveries made at key moments of invention.

Such approaches invite technology, and in particular computing curators, to refocus the stories they tell in museums, and place users rather than innovators at the heart of the story. But what does this mean for visitors, who expect to hear about the firsts, the Eureka moment, lone engineer who invented a new approach? I would argue that if done well, such an exhibition can be more engaging, telling stories of users as active co-producer, showing how the technology of computing is enmeshed in the culture through which it was produced and used. Rather than one significant individual, we begin to learn about teams of people working together to define a technology. The marketing materials that illuminate how the user and the non-user of new technologies were defined become vital, rather than relegated to the depths of 'trade literature'. And oral histories, that present an unofficial and often undocumented historya 'history from below'give a voice to many of those developers and users whose tales of technology are otherwise unheard. These can provide a rich resource for a deeper, more personal connection with technological histories and in particular with specific objects whose relevance and use might be lost.

Of course there are negative implications to a story telling approach. By their very nature, stories aren't always true and can often heighten the sense of adventure, tragedy or comedy in order to play on the audiences' emotional engagement. A story telling approach suggests a shift away from museums presenting 'factually correct' history, to the presentation of history that plays on audience feelings, presenting real historical figures as merely characters in a plot. It also presents the question; if a museum is about stories, can it also be about ideas? If the craft of a curator is to play with the experience of visitors, orchestrating moments of calm and reflection with moments of excitement and emotion, can it still invite visitors to make connections between objects and absorb ideas?

The Role of Objects

It is often said that for information technology, working machines are key to a museum display. Many believe that there is nothing less interesting than a dead computer, as the information and the interface are all lost from the Some visitors want to look at a machine and understand 'how it works', others want to hear the noise and see how the machine was run. But technological objects can offer different types of museum experience to audiences. An object (even an apparently historically banal object) can have a power for visitors just in themselves. To focus on working machines only is to disregard the different type of experiences that an object can provide. It also disregards the craft of the curator in layering and texturing the experiences across the gallery.

In his essay 'Resonance and Wonder', the literary historian Stephen Greenblatt analyses the museum experience in terms of two types of audience response. "Resonance" is "the power of the displayed object to reach out beyond its formal boundaries to a larger world, to evoke in the viewer the complex, dynamic cultural forces from which it has emerged". For Greenblatt this is essentially the knowledge and understanding provided by that object, if viewed through a cultural or historical lens. This could be the object as a working machine, or it could be the object with merely a label, inviting it to be considered in its wider context. It is about understanding the uses and instances which have created a meaningful frame for the historical artefact. In Greenblatt's words, it is "the power of the displayed object to reach out beyond its formal boundaries to a larger world".

In contrast Greenblatt's term "Wonder" refers to the object's aesthetic and poetic dimension, the power of the object "to stop the viewer in his or her tracks, to convey an arresting sense of uniqueness, to evoke an exalted attention". Greenblatt was originally referring to the arts and visual artefacts and some struggle to see how this sense of "Wonder" applies to scientific and technological artefacts. But computer and other information technologies have the power to command visitors' attention, even when their meaning or use is not commonly understood. The scale of a supercomputer computer, such as the Cray 1 from 1976, can provide visitors with a sense of awe from the bold theatrical design, or the magic of seeing "the real thing", achieved by icons such as the Apple 1 from 1976, can give visitors and exceptional sense of history. Icons such as Charles Babbage's Difference Engine 1, with its intricate cogs and wheels, bring amazement that one individual could make such a leap of imagination.

It is the curator's role to amplify the experience of "Resonance and Wonder" through the stories that they choose to tell, the objects they display and the way the gallery is designed. Some significant objects might be presented as "design pieces" without any context or story, but purely to invite audiences to relish in the real or the beauty of the object. Others might be part of a narrative that engages the visitor emotionally in a story. A working object might focus the experience on to one type of understanding, "what did it do and how?", whereas carefully crafted locations of objects can allow visitors to make serendipitous connections, inviting the audience to think for themselves and construct their own meaning between objects.

Narrative in the Information Age Gallery

Opening in autumn 2014, Information Age will be based on the second floor of the museum, in what was the old shipping gallery, which is a vast 2500m 2 . The gallery tells the story of the transformations that have taken place in human communication in the last 200 years. Blending historic, iconic objects with up-to-the-minute technology and interactive media, it will expose, examine and celebrate the technology that has changed the way we share information and connect with one another.

Starting with the development of the electric telegraph in the 1830s, the gallery will tell stories through the eyes of those that invented, operated, and were affected by each new wave of technology. It is not a uniquely technological story, but a social story of the successes and failures, the brave new ideas and ambitious schemes, and of human nature and our universal need to connect with each other.

The gallery is constructed around six zones that reflect networks of people, technology and organisations: The Cable, looks at telegraphy, The Broadcast looks at radio and television; The Exchange focuses on telephony, The Constellation looks at Satellite communications, The Web looks at computing networks, and The Cell focuses on cellular mobile networks.

Each zone, or network, highlights 3 or 4 "transforming events" that are significant stories that enable visitors to experience a key moment of change through technology. A total of 21 transforming events across the gallery place the stories of users equally with those of the inventors of technology. Some present a key innovation e.g. Alexander Graham Bell's development of the telephone, but in this case the story of invention is contrasted with Bell's love story and through his competition with Elisha Gray.

In the section on computer networks, The Web, there are four stories. One story looks at the development of the world's first computer for commercial applications, the Lyons Electronic Office (LEO 1) in 1951. This doesn't just focus on the story of those who created the machine, but will highlight the importance of research and influences from a range of actors across Britain and America as knowledge about digital electronic machines is shared and developed. The teashop manageresses will also play a central role, showing how they became as much part of the 'information system' by calling in their weekly orders, as the LEO computer itself. We hope that this type of story and user-centric approach will create personal links for visitors and enable a dialogue with a broader audience than those already engaged with the history of computing. It should enable visitors to reflect on what the technology meant to our predecessors, and how these meanings are not given and defined by the technology, but are co-constructed with users in the way that they are consumed, marketed, used and re-appropriated.

The approach has radically altered the way the team is working on this project. It has meant that oral history and in some cases video, has become both a central tool for the research process. These multiple types of interpretation will form a very important part of the engagement of visitors on gallery, giving personality and historical context to the story, and in the longer term much of the material we collect will become a long-term digital asset within the museum's collections.

We also aim to play with Greeblatt's concepts of resonance and wonder through significant gateway objects. We have chosen particular objects for the awe and spectacle they provide to visitors. These will not be heavily interpreted through the stories, but we believe they will give a meaningful context, provide a visual pleasure and a historical anchor that can only be achieved by museum artefacts.

An example of this is the vast Russian supercomputer that will be placed in dialogue with a comparable machine from the USA, the CDC 6600. Although we only have space to display the main operating console, the scale of this machine and surprise of seeing the only Russian supercomputer from the Cold War in a Western museum collection should provide visitors with a real sense of astonishment.

Conclusion

The craft of the curator is in layering and texturing different types of experience across the gallery. Scientific and technical museums have tended to focus on one approach to history -chronological accounts that tell familiar technical stories of invention and progress. These appeal to a core audience who are already engaged with the history of computing, but provide few ways in to those who do not think computer history has relevance or interest for them.

The display of the history of computing, and more specifically the history of information machines, as a series of narratives that focus on the use of the machines, as much as the invention, provides museums with a real opportunity to broaden the audience for the history of computing. A people-centric approach, that sees the history of computing as a lens on society and humanity, helps to connect visitors with this technical history at a more personal level.

But where does this leave the artefacts of computing; the historical evidence of the material culture of information processing and storage? Working machines can play an important role in helping visitors to understand the experience and environments of computing, but they should not only be considered as working tools. Greenblatt's appeal to the Resonance and Wonder of objects can be equally true for scientific and technological artefacts. If given the platform to perform, objects not only enable us to reflect on social and cultural forces, but to see the poetic and aesthetic dimensions of the machine and its design. All of these devicesour stories, users and objectsare the tools we are working with in the development of Information Age gallery.

Figure 1 :

 1 Figure 1: Science Museum's gallery Computing Then and Now, 1975, showing the computer terminal that allowed visitors to interact with the Imperial College computer.

Figure 2 :

 2 Figure 2: Map of the Science Museum's gallery Computing Then and Now, 1975.

  The other stories look at the Birth of Computer Networks: How the world's first international computer network -ARPANETdeveloped out of Cold War tensions in 1969; A Global Information Space: The creation of a new age by Tim Berners-Lee in 1990, where anyone can find and access information through the World Wide Web; and Computers for Users: The transformation of computers into intuitive and affordable devices in 1983 through the development of the Graphic User Interface and the launch of the Apple Lisa computer. In each story the users are actively involved in defining the meaning of the technology and the way it fits into existing social structures.

Figure 3 :

 3 Figure 3: Universal Design Studio (the gallery designer's) representation of the network structure across the Information Age gallery, 2013


Norbert Ryska

Jochen Viehoff
email: jviehoff@hnf.de

Keywords: Computer museum, Nixdorf, Nixdorf Computer AG, simulations, replicas, designers, builders, education

In the late 1970s Heinz Nixdorf began to collect historical calculating devices and early computers to serve as the basic exhibits of a computer museum. After Nixdorf's death, the Nixdorf Foundation was set up to develop the museum. To see the different ways of looking at computer history, a pluralistic approach was chosen, with the focus on the history of objects, ideas, people or societies. This paper tells something of the development, goals and purpose of the museum.

1 Prologue: 1975 -1992 It was in the late 1970s that German computer entrepreneur Heinz Nixdorf began to collect historical calculating devices and early computers. These objects were to serve as the basic exhibits of a computer museum which he intended to establish on his Paderborn premises. Not only had Nixdorf collected some 1,500 objects by the time of his sudden death in 1986, but he had also been presented with an initial exhibition scenario by the Berlin exhibition architects.

Development Phase: 199-1996

Following a six-year hiatus, the idea of the museum was revived by the Nixdorf Foundation and a ten-strong working group comprising technology historians, exhibition architects and one of the authors (Ryska) was established with a view to performing a feasibility study. The contextual framework stipulated by the Nixdorf Foundation was initially restricted to the history of office computing in the light of the product range manufactured by Nixdorf Computer AG. However the working group opted to dispense with this narrow outlook, instead developing an extensive tour through the 5,000-year history of arithmetic, writing and communication. This idea, which ultimately cost DM 120 million, was finally sanctioned by the Nixdorf Foundation. The concept was absolutely unique.

The following objectives were pursued and achieved in planning the permanent exhibition on the "History of Computing" which was to be set up over 6,000 square metres of floor space in the former headquarters of Nixdorf Computer AG:

Concept and Goals

The working group identified 60 topics to be elaborated by technology historians from around the globe and local curators before being implemented across two 3,000square-metre storeys.

 1st floor: From the abacus to the computer (3000 BC -1950)  2nd floor: Computers conquer the world (1950present) A chronological presentation of the "History of Computing" was preferred to a diachronic approach, i.e. a "back to the roots" or "from the roots onwards" perspective.

In a bid to get to grips with the different ways of looking at computer history, a pluralistic approach was chosen, with the focus on the history of objects, ideas, people or societies, depending on the topic in question.

To avoid having to forgo more complex yet important issues, it was decided that the individual topics could certainly call for different levels of understanding, in line with the principle that not everyone is interested in everything in any case.

The objects, contents and presentations should be "unique", i.e. not be available in the same form anywhere else in the world.

In a bid to render the museum experience more memorable for visitors, exhibits should, where possible, have a "back story", whether this be the curious tale of the Chess Turk, Hollerith's use of the punched card for data processing, the fact that Friedrich Nietzsche typed on a Malling Hansen, Zuse's construction of the Z1 in his living room, how Curt Herzstark's Curta calculator was developed in a concentration camp, the cracking of the ENIGMA code at Bletchley Park and Moore's Law as the only "law" in IT history.

An important objective for a public simply wallowing in superlatives was of course to procure "exhibits with cult status" (firsts, rarities, exotic models) such as the Apple I, Altair, CRAY 2, Enigma, computers used in space and Heinz Nixdorf's first computer, to name just a few examples. In the case of IT products such as calculators and mobile phones, their range and diversity was also to be demonstrated.

The successful procurement of significant loans such as components of the ENIAC (NMAH), the Typex encryption machine (Science Museum London) , the Geheimschreiber machine (Siemens AG), the D11 tabulator (IBM), the on-board computer of the Gemini Space Capsule (Air & Space Museum) and encryption devices (NSA) is evidence of the importance attached by international lenders to computer history.

What about Stories about the Designers and Builders of these

Computers?

A central "Hall of Fame" (15 people) and an electronic "Wall of Fame" (152 people) expressed the concept that man was to serve as the focal point in the contemplation of his dealings with technologyin his role as a scientist, inventor or entrepreneur.

Using Simulations and Replicas to Illustrate the History of Computing

From the outset, content planners endeavoured to create an even balance between hands-on functional models (some 30 in number) and multimedia methods of communicating information on technology (around 30 multimedia presentations). For the first time, the computer became an instrument used to impart information on its own history.

The idea was to demonstrate the inventive spirit and technological competence of earlier ages by means of numerous replicas of historical calculators and machines.

Problem Areas

In researching historical computers, we came across surprisingly little awareness of tradition, particularly in the IT industry itself. Even "classic devices" were often no longer available (in sufficient quantities) at their manufacturers. The "last of the Mohicans" included Digital Equipment, IBM, Siemens, Bull and Ericsson. Philips, for example, closed its spectacular company museum Evoluon in Eindhoven in 1989 in the midst of a financial crisis.

The central exhibition area "How does a computer work?" was implemented for the first time in 1996 and completely revised in 2003. Both approaches fell short of our own and visitors' expectations. It is still difficult to illustrate clearly how a computer works by means of analogies or modellingespecially, of course, in the case of software. The HNF nevertheless created an exhibition section entitled "Software and computer science" in 2007 in an attempt to remedy these deficits. The HNF is the first museum ever to address this topic and has now been presenting it for ten years in a variety of ways.

As of 2014, it is planning an IT_Lab to impart the basics of computer technology in interactive fashion in the style of science centre exhibits.

Motivation

In the future, the museum both can and should play an important role as an extracurricular educational resource for the imparting of core skills in areas such as the natural sciences and technology and in the provision of experience-oriented learning environments. In this context, the successful special exhibitions at the HNF demonstrate clearly the value of interactive and multimedia exhibits in the modernday transfer of knowledge.

The objectives in updating the permanent exhibition are to preserve the fundamental character of the venue as a museum for information and communication technologies, while meeting the new expectations of future generations of visitors.

Concept

As part of the planned renovation and updating of the permanent exhibition (Update 3), the "exit area" on the second floor is to be extended as a laboratory for scientific experiments in the field of information technologies. To supplement the sections "How does a computer work?" und "Digital workbench", clusters of hands-on exhibits will enable visitors to work on basic scientific aspects from the fields of electronics, computers and the media interactively in a "laboratory situation".

Unlike "conventional" science centres, the Laboratory for Information Technologies has a clearly defined contextual focus on topics relevant to the HNF. Indepth levels of information as well as the reference to objects that is so characteristic of the HNF are also planned for each individual section.

The modular topic islands allow the integration of current technology trends almost as they happen. Moreover, the focus on the scientific and technical basics will considerably extend the duration of the cycleand thus the topical referencevis-àvis device developments and application ideas.

The new laboratory unit geared to basic aspects can be advertised in targeted fashion as part of marketing measures with a view to attracting more groups from schools. A "science centre" in the museum context oriented clearly towards the information technologies in terms of topics would furthermore constitute another unique feature of the HNF.

Planning Stages

 Stage I: Five stations with basic scientific aspects of information technologies  Stage II: Five stations with other topics relevant to the HNF Completed project: 10 stations each with 3-4 laboratory workplaces for scientific hands-on experiments oriented towards basic aspects 10 Main Topics Bits and bytes, electricity, microelectronics, calculation and logic, peripherals, semiconductors, data storage, GPS, networks, nanoelectronics, computers of the future, energy, green computing, digital media, mobile communications, robotics

We have discovered over time that current IT devices which are generally already in private use attract relatively little attention in the museum. Due to the extremely rapid pace of development in the industry, many of these devices become "obsolete" within a few years, if not months in some cases. A period of at least 40 years then elapses before these devices are once again looked upon with some degree of curiosity by visitors from the younger generations. In the meantime they are largely regarded as nothing more than electronic junk.

A museum is also no longer capable of reproducing the vast variety of products available in today's ICT industry. What's more, a serious evaluation of technologies and products can only take place once several years have elapsed.

The maintenance of early computers in particular (1950s-1980s) has proved and continues to prove impossible or unfeasible on conservational grounds in many cases, due to the frequent difficulty of obtaining spare parts, a lack of software and insufficient know-how.

Fortunately, the number of simulations of historical computersof particular interest to fans of retro computersis rising steadily. Some of the historical operating systems of home computers implemented by the HNF in its PC gallery and elsewhere via emulators include the Apple II, C64, CPM, MS-DOS 1.x, 2.x, 3.x, 5.x, 6.x, Windows 1, Windows 2, Windows 3.11 and Windows 9.x. Retro computing can be traced back to a longing to return to the origins of information technology; it also offers the opportunity to work with hardware at close quarters and to "have a say" at systems level. It is easiest to explain the basic principles of technology with reference to historical systems, as the latter are not overly complex.

The Importance of Education in Making History of Computing Relevant

HNF has compiled a varied educational museum programme to motivate children and young people to take an active approach to the exhibits and their history. At workshops children can, for example, build robots, encrypt messages or learn how to 'make' paper. Teachers and pupils are given numerous ideas for study content.

Besides a guided tour of the permanent exhibition, special tours can be booked on such topics as arithmetic, writing, inventors and entrepreneurs, women's work in information technology, cryptology and the history of communications.

12 Trial phase (1996 -present day)

Numerous activities have taken place in a bid to further popularise the "History of Computing" since the HNF first opened in October 1996. Only the biggest projects are named here. In 1998In , 1999In and 2001, the HNF joined forces with Westdeutscher Rundfunk 3 to stage the "WDR Computer Nights", a 7-hour live TV marathon attracting 3,000 visitors to the HNF as well as hundreds of thousands of television viewers.

The permanent exhibition was updated in 2004 with the addition of new themes such as robotics and artificial intelligence, mobile communications and digitization. The new galleries present the latest information technology themes in an interactive, multimedia exhibit. Visitors can try their skills at old and new computer games, test advanced man-machine interfaces and experiment with the latest applications and products from research and industry in the showroom

In 2004, the big special exhibition "Computer.Brain" offered a comparative presentation of these two thinking tools. Significant fields of application of the

The Heinz Nixdorf MuseumsForum, Central Venue for the "History of Computing"

computer were subsequently also presented in the form of "Computer.Medicine" in 2007 and "Computer.Sport" in 2009.

The relevance of the "History of Computing" for the public is ultimately also clear from the HNF visitor figures: over 16 years, the museum has registered some 1.9 million visitors, 36,000 guided tours and 13,000 events, as well as generating a huge media response (400 million readers and viewers/listeners ??) and a consistently high level of appreciation from within the computer industry.