Operating efficiency increases with the processingpower of a computer, and therefore there is acontinuous market demand for larger computers.Meeting this demand poses difficult design problemsfor the computer manufacturer which can be solvedmost efficiently by using automation techniques.In this article, the author describes the automatlon .techniques employed in the design and manufactureof International Computers Ltdo's 1906A computer.

Automation of the designand manufacture of a largedigital computerby C. D. MAR SH, M.A., C.Eng., F.I.E.E.

IT IS GENERALLY true that the greater theprocessing power of a computer, thegreater its efficiency in terms. of cost percomputation (assuming that the formid­able problems of feeding a large com­puter with an adequate flow of work aresuccessfully dealt with).In addition, as- experience and con­

fidence in computing have grown, there hasbeen an insistent demand for the solutionof single problems which, while being toolarge for computers currently in use,could not successfully be partitioned intoa number of smaller problems.

As a consequence, there has arisen acontinuous requirement from the marketfor central processors which are bothfast and large. The 1906A computerspecification was a response to such ademand.

The ICL 1906A specification posedsome particularly difficult design prob­lems. In order to meet the fast instructionexecution times called for, it was neces­sary to develop, in collaboration with anAmerican semiconductor company, afamily of integrated logic circuits with atypical stage delay of 2ns, which was, initself, an important exercise in inter­national technical collaboration.The complexity of the specification

involved the connecting together of"40000 of these fast logic gates by 250000connecting wires. Since the speed of pro­pagation of electrical signals in an insu­lated wire is only about 1ft in 2ns, theintrinsic speed of the circuits can only beutilised if the average length of the con­necting wires is kept to a few inches. Inspite of the fact that the central-processor

1 Cutawayviewof 1906Acomputer, showing the logic circuits housed in the swinging doorframesElectronics & Power October 1970

cabinet measures 6ft 4in high by 3ft 8inwide by 27in long and its 800 cableformscontain 11 miles of coaxial cabling and 17miles of 'twisted-pair' cabling, criticallengths of logic-interconnection wire havebeen kept down, on average, to a lengthof 3in (equivalent to a delay of only0·5ns).

Furthermore, in order to avoid losingtime in the interconnections because ofmultiple reflections travelling back andforth along the wires (,ringing'), all inter­connections had to be made in the formof correctly terminated transmission linesof constant impedance. This requirementinvolved the development of a multi­layer interconnection plane ('12-layerplatter'), combining stripline techniqueswith a density of interconnection nothitherto attempted anywhere in commer­cial equipment.The degree of precision and mass of

data required in the manufacture of thesemultilayer interconnection platters de­manded, in turn, the use of automaticartwork generators, numerically con­trolled drilling machines and automatictesting machines. The input to all thesenumerically controlled machines takes theform of punched paper tape or magnetictape produced by the design-automationsystem.The problem of designing and building

these multilayer interconnection planeshas demanded a major effort, not only incomputer-aided design but also in processtechnology allied to precision engineering.The design-automation system has been

developed as a means of turning thedesigner's pencil sketches of the detailedlogic of a computer into complete manu­facturing information and documentation.A suite of simulation programs called

SIMBOL is used first to check the consistency

Th'is article is based on a lecture given by C. D. Marsh,G. Haley and G. Adshead to the lEE at Savoy Placeon the 13th March 1970.

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of the basic overall system designand to allow it to be subdivided intosmaller units that can each be handled byseparate logic-design teams.

The design-automation suite of pro­grams then operates at the detailed logiclevel, checking that the circuit-loadingrules have not been transgressed and thatthe logic chains are complete. The pro­grams are used to work out the optimumphysical location ·for each logic elementon the interconnection platters, afterwhich the positions of the wiring runs aredeveloped. When this latter phase is com­plete, the manufacturing information canbe produced. This output takes the form ofpunched paper tapes to drive the artworkgenerator, the numerically controlleddrills and automatic tester, and the line­printer output to provide documentationfor production, test and maintenanceusers.

The system keeps an accurate controlover the vast amount of data generated inthe course of the design process, and hasmany built-in checking procedures thatallow the computer to warn the designerof errors and omissions in his basic work.Once these errors have been corrected, thecomputer can manipulate the data todevelop the physical basis for the design ina rapid and error-free manner. This job is

. too time consuming and complex forhuman beings to carry out successfully onsuch it large scale as is required for thiscomputer.

Hardware systemFig. I shows the 1906A computer.

Three bays house the logic circuits in theswinging door frames, while the cabinetsat each end provide cooling and filtrationfor the closed-circuit air-conditioningsystem. The row of cabinets at the rearhouses the power-supply system. Thememory system, which is not shown,occupies a third cabinet.The integrated circuits used are a

special type of emitter-coupled logic(e.c.i.) on single silicon chips, with up to100 components (30 transistors, plusdiodes and resistors) per chip. The 2 x1·5in plug-in modules, of which there areseven basic types (Fig. 2), carry the inte­grated circuits together with the thick-filmmultiple-line-matching resistors. Certainnonstandard functions, e.g, for voltage­level conversion at interfaces, are made byhybrid techniques using a multiple-tran­sistor fiatpack and thick-film resistors(shown unencapsulated in Fig. 2).20000 of these circuit mod Illes are

conceptually plugged into a continuousmultilayer interconnection plane 140ft2•in area. In practice, the largest plane whichcan conveniently be made is 13 x l6in­large enough to carry 200 circuit modules,of which 94 (Fig. 3) are used in the com­puter. These planes are called 'platters'and are connected together (six or nine ata time) along their peripheries to form alarger plane by bridging connectorsclamped on to the gold-plated pad areas.Signals between door planes an! carriedby cableforms of coaxial cable or twistedpairs. The complete machine uses over800 cableforms, nearly half of which areused for engineer's monitoring purposes.

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2 Unencapsulated·integrated-circuit mod­ules using special Motorola e.c.1.on singlesilicon chips

3 12-layer platter loaded with circuitmodules

Inside the platters, the signals arecarried between modules by the printedwiring on the two pairs of logic planes(Fig. 4). These planes are buried withinthe platter, accurately spaced from anearth plane to give a 75Q-impedancetransmission-line characteristic. Anorthogonal system is used. X and Ytracks are connected by 0·013in-diameterholes drilled in 0·030 in-diameter pads andthrough-hole copper plated. Theseconnections are called 'via' holes. Theplane carries about 1000 connections affdrequires 6500 accurately drilled holes.The X wires are 0·007 in wide and the Ywires are O'OlOin wide. (Spacing from theearth plane is 0·012in for the X wires and0·021 for the Y wires.)

Fig. 5 shows a cross-section of a bonded12-layer board showing the via holes.On the left is a module pinhole goingright through the board. The two outerlayers merely contain pads for solderingthe module connectors. The centre fourlayers carry various power supplies,

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which are isolated from the module exceptwhen it is desired to make a connectionto a power rail. On either side of the powerlayers are two groups of signal planes.Each group contains an earth plane forimpedance matching and an XY pair tocarry the logic signals. Some X and Ytracks are shown together with someinternal via holes.

Artwork]t will now be useful to present a broad

picture of the production processes in­volved in making the multilayer inter­connection platters.The patterns to be etched on the various

copper layers of the platters must betransferred from photographic' negatives,known as 'artwork'.

High-accuracy artwork is produced bya precision XY plotter. This XY plotterhas a moving light source, controllednominally to within ±O'OOIin overthe bed from a magnetic or punchedtape. The light beam, controllable in widthand intensity by program, plots the art­work at full scale. In practice, mostartwork consists "Of a standard patternplus a variable part. To save plotting time(and expense !), the film is pre-exposedwith the standard pattern and only thevariable additions are plotted. Thevariable parts of each platter design need0·25million characters of paper-tapeinput to the plotter.All process stages involving the genera­

tion or use of artwork must be carried outin a dust-free environment controlled towithin 50 ± 5% relative humidity and70 ± 2°C temperature.Before applying the artwork to the

copperclad laminates, the latter have tobe coated with a light-sensitive coatingknown as photoresist. Most printed­circuit production units use the wetapplication process, but, for the finest­tolerance work on the logic layers, a dry­film method has proved to be essentialto eliminate pinholes and blemishes.

Layup of platterAlthough there are 12 etched copper

layers in the platter (Fig. 6), they areassembled from seven laminates (fivedouble-sided and two single-sided). Thelaminates are separated by spacers im­pregnated with half-cured epoxy resinduring assembly in the accurately tooledlaminating jig. The assembled layers are :heated to 165°C under a pressure of3001bf/in2 in a steam-heated bondingpress to cure the epoxy resin and bondthe laminates (Fig. 6).The 5000 major via holes linking the

layers together are drilled (after bonding)on a 16-spindle numerically controlleddrill, which has a nominal accuracywithin ±0·002in.

In total, for each platter, 18000 holesmust be drilled in glass-fibre laminate toan accuracy of within 0·002-0·003 in. The6500 via holes in each logic layer have tobe drilled before bonding. Drill wear is amajor problem and drill consumption is650 per week.

At various stages during manufacture,it is necessary to test the layers, and amachine has been specially developed todo this automatically. Controlled by

Electronics & Power October 1970

punched paper tape, this machine checksfor open- .and short-circuit conditionsbetween all possible pairs of points, andprints out the location of any errors found.The printed-error slip is attached to anyfaulty work and sent to the rectificationsection.During the development of the fore­

going production techniques, many diffi­cult technical problems have had to beovercome. In particular, one can note thealmost incompatible nature of many ofthe processes, as the two followingexamples show: artwork generation andhandling demand a closely controlleddust-free environment, while the associa­ted stages of drilling and sanding generatea great deal of dust, swarf and fibres;accurate registration of the layers de­mands physical stability of the basiclaminates, but these are subject to asuccession of wet and dry chemical pro­cesses, culminating in an extremely brutalbonding process. The investment in plantand equipment to carry out these taskshas totalled £0·75million over three years.

Computer·aided·design process'The first main process of design in­

volves breaking down the planning·specification into a system structure(Fig. 7). This means defining a set ofblocks of manageable proportions (such'as a floating-point-unit, store contro!),each block being made the responsibilityof a group of logic designers, who go onto construct the inner details of the logicconnections.The computer-aided design falls into

two broad categories. First, the simulationlanguage SIMBOL is used to specify. thesystem structure and order code and tosimulate real programs. This operationis to prove that the system structure isviable and that the detailed logic of eachblock agrees with its system specification.Secondly, the other group of programs

forms a design-automation suite knownas SYSTEM.67. This suite is concerned withthe problem of converting detailed logicinto a physical form, deriving the inter­connection patterns and generating allthe . relevant production-informationdocumentation. An 'information expan­sion' is associated with the process. Inthis case, a product specification of10000 characters of information is ex­panded into 200million characters ofproduction information.Once the logic engineer has completed

his system-design studies with the aid ofthe simulation program, he is left with aspecification of a group of logic. Thislogic group could be, for example, afloating-point unit occupying nine plat­ters. Ultimately, this group will be definedin detail on 80 sheets of logic drawings.The logic designer roughly sketches out

the basic logic. This sketch is then passedto a technical clerk who codes the logicalinformation into the computer inputlanguage. This information is thenpunched on paper tape and fed intothe computer. Alternatively, the logicdesignermay describe his logic in the formof Boolean equations without drawing alogic sketch. Where there are repeatedslices of logic, various MACRO facilitiesElectronics & Power October 1970

4 Pair of logic layers for carrying signals betweenmodules inside the platter

outer module pad

monitor or power logic layer

5 Cross-section of a bonded12-layerboard showing the 'via' holes

6 Lay-up of a platter377

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may be stored in the computer and calledup as requested to minimise the datainput.The main elements of the system are

represented in Fig. 8. All information isinput to the logic files in the form ofmodifications. Even brand-new drawingsare considered as a modification to anempty file. This is a very fundamentalpoint; it ensures that all possible ramifica­tions of any change are considered.Documentation is always kept in step,and any errors are indicated at the timeof the modification. .The engineer can ask for about 40

different types of information printoutfrom the files. Perhaps the most usefulprintout is the error listing, which listserrors or inconsistencies in the design dis­covered by the computer's checkingroutines. If the designer has broken therules, he soon knows about it! Thesecomputer programs can detect over 30different classes of logic error that mayexist on the file.One of the major design processes is

'placement', or the problem of decidingthe location of the logicelements through­out the computer. Each logic elementmust be associated with a particular cir­cuit on a particular plug-in module on aparticular platter.

Basic choiceThe basic partitioning choice of which

groups of elements are to go on whichplatter is made by the logic designer;thereafter the computer takes over. Thecomputer program then considers 500-~OO elements at a time, and collectstogether groups of similar elements thatcan form one plug-in module. Thisgrouping is achieved by scanning roundthe logic diagrams for similar elementsthat have been drawn close together andfinding the most interconnected clusters.An interconnection matrix is then set

up, storing the links between the modulesthat are candidates for the 200 modulepositions. Several cycles of various al­gorithms involving such manipulativetechniques as 'elastic contraction' and'pair swopping' are used, and eventuallya fairly good layout of modules isachieved.Once the physical placement is com­

plete, the interconnection paths can bedeveloped; this is called 'tracking'. Thetracking operation is the job of fitting upto 2000 wires into the two logic layers ofthe multilayer board. An exhaustive'maze-running' technique is employed(based on Lee's algorithm), which willonly reject a wire if neither plane canpossibly accommodate it. In most cases,only a few wires cannot be accommo­dated, and these are added as hand wiresin the final production stages.From the production file, a large quan­

tity of numerical-control tape is generatedto drive the X Yplotters, drillingmachinesand platter tester, and also to generateassembly- and production-control docu-mentation is printed. 'Throughout, ,all the files store informa­

tion in a form which preserves previoushistory. Consequently, any drawing ordocument may be reproduced as it378

productspecification10k characters

/ \system structure-- / \detailed logic

/ \interconnections design/ I "<, automation

manufacture test documentati~n I(200 millioncharacters)

standarddata I

sirnufction

=htechnology

production

documentation

7 Design process

8 Design-automationsystem

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9 Calcompplotter drawinga logic diagramonline from a computer

existed in the past, and, more importantly,the changes to any document betweenany two dates in the past may be deducedand printed out when required.A graph plotter attached to the com­

puter draws the finalised logic diagramsdirectly from the computer files (Fig. 9).The drawing produced is used by thedesigner as a record of his work, and alsoby the systems test and maintenanceengineers. The diagrams give an accuratepicture of the version of the computerthat is actually being built. Each weekabout 200 high-grade master drawingsand 300 design copies are produced.

During the course of various opera­tions, the programs need to refer to hard­ware constants such as dimensions, pinnumbers of modules etc. All these data,which are fixed for a particular project,are kept on sets of tables.' Each file has ahardware code definition which allowsthe programs to refer to the relevanttables. To date, over ten types of hard­ware models using completely differenttechnologies have been constructed usingthe same set of programs. These programstotal nearly 0·5 million instructions whichare held, togetherwith the tables, in an over­lay structure on disc or drum memories.

Electronics& Power October 1970

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10 Computer operations room

About half this total of instructions(200000)was needed for the design of theparticular machine under discussion. Therunning of these programs is controlledby a special operating system allowingmixed__batch_processing and__onlineoperation.

Design operations roomThe design of each platter, together

with the issue of all production informa­tion, takes about 20h of computer time.In addition, there is the simulation load,other projects and program developmentto consider; a complex of three large com­patible processors with suitable peri­pherals is dedicated to c.a.d. problems,and 4000reels of tape are in use. The totalcapital investment in design automationat this location is about £i million.These machines are run 24h a day on a

3-shift basis. Great emphasis is placed onreducing turn-round time, and very fewjobs take longer than 24h. Any designengineer handing in a new logic sketch at,say, 4 p.m. would expect full feedbackfirst thing next morning. Some of thecoding and data-preparation operationsare performed on the evening shift, andthe jobs are run on the night shifts. Atpresent, an online job-control system isused to keep track of over 1000 jobsa week. In this context, a 'job' con­sumes between 3min and 2h of computertime.The design-automation department has

a staff of about 50, and half of these areinvolved in the operating, data-control,data-preparation and coding areas. Theremainder are engineer/programmers in­volved in generating new programs andimproving the current ones.It is difficult to define the programming

development effort because there is aworld of difference between producingElectronics & Power October 1970

the first programs capable of designing aplatter and producing a working pro­gram suite consuming hundreds of hoursa week of computer time. In many prob­lem areas, it is difficult to say where the.engineering leaves off. and the program­ming starts. The current library ofprograms of 0·5million instructionsrepresents the writing of about 40-50instructions per man per day over the lastfour years; but the general developmentgoes back over seven years or more.One aspect of the system described can

be called its 'amplification ratio'. For mostareas of the machine, about 200charactersof output are generated for each designcharacter input by the design engineer.Where there were large repeated slices oflogic, this ratio has reached as high as7000: 1.

Benefits from automationAccuracy of design information is of

particular importance in a digital com­puter. The logic paths are so complex thata large computer never operates twiceunder exactly the same conditions. In thepast, minor errors in wiring in even asmall computer have not shown up duringmonths of rigorous testing, but have­come to light after a year or more ofoperation at a customer's Site, when aparticular combination of input data andprogram steps has occurred.Consequently, it is of fundamental

importance to check the logic designthoroughly. When one bears in mind thatthe design information of the 1906Aamounts to 200 million characters, it willbe realised that manual methods ofchecking and data transcription couldnot be contemplated.The use of a computer simulation

technique, followed by a computer check­out of the detailed logic design, reduces

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the possibility of an undetected designerror to a negligible value.Owing to the complexities of a com­

puter, changes in the logic design areinevitable as development proceeds. It isimp-ortant to be able to process designchanges quickly and generate new manu­facturing information and its attendantdocumentation simultaneously; and thishas been one of the prime aims in thedevelopment.

Manufacturing-technique accuracyThe importance of generating and

maintaining error-free design informationhas been stressed earlier. It is equallyimportant to ensure that the computeris built exactly to the design information.Wrongly placed wires have been a costlyhazard in computer manufacture formany years, but, with the high frequenciesinvolved in nanosecond pulses, the length,positioning and screening of the wiresare equally important.An automated method of manufacture

ensures that each computer built is notonly interconnected correctly, but that itis exactly identical to the prototype in theimportant minor details of wire lengthand positioning. This fact, when coupledwith the rigorous pretesting of the plug­in circuit modules, leads to a short andpredictable system commissioning cycle.The benefits of this time reduction, interms of predictable delivery dates andminimal work in progress, are obvious.But the savings accruing from a 'get it

right first time' approach cannot easilybe measured in financial terms. The gainin electronics reliability, and generalincrease in confidence in the quality of thedesign and manufacture, are powerfulfactors in gaining the rapid market accept­ance that is so important in selling largecomputer systems in the world market.

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