Hybrid ComputerSystemsGuest Editor's IntroductionRobert M. HoweUniversity of Michigan

Today's increasing emphasis on simulation ofcontinuous dynamic systems lends a special signi-ficance to this issue of Computer. Hybrid computers,with their tremendous computing speed resultingfrom the all-parallel configuration of their analogsubsystem, are particularly suited for dynamicsystem simulation. Indeed, back in the 1950'sanalog computers were the only game in town whenit came to real-time simulation of flight vehicles.Digital computers of that era were simply too slowin the numerical solution of differential equations.The first generation of hybrid computer systems

appeared in the 1960's, when digital computerswere combined with analog computers. Digitalcomputers provided storage and logic capabilityalong with increased arithmetic power and precisionto augment the continuous integration capabilityand speed of analog computers. The resultinghybrid computers were capable of expanded real-time operation and even permitted faster-than-real-time simulation of dynamic systems-a highlydesirable feature allowing cost-effective iterativesolutions of optimization problems, partial dif-ferential equations, and dynamic stochastic prob-lems. As second- and third-generation hybrid com-

puter systems have evolved, the digital computerhas been used more and more for high-speed setup

July 1976

and checkout of the analog subsystem as well asthe performance of specialized computational tasksnot easily accomplished on the analog, such asfunction generation.The five papers in this issue emphasize both

present and future directions of hybrid computa-tion. The first paper, by Paul Landauer of Elec-tronic Associates, Inc., is largely tutorial and de-scribes typical current hybrid systems, how theyoperate, and where they are applied. Cost effective-ness of hybrid computers versus all-digital com-puters in solving dynamic problems is emphasizedin this paper.The second paper, by Dr. Harriett Rigas of

Washington State University and currently onleave at the National Science Foundation, dealsspecifically with hybrid computer software systems,including a discussion of hybrid compilers. Thesecompilers use a source program in a Fortran orsimulation-language format, automatically trans-lating this into a hybrid object code, includinganalog component assignments and patching lists.Such compilers greatly increase the ease of pro-gramming of hybrid computers.Next a paper by Landauer and myself presents

a quantitative method of comparing the speed ofhybrid and all-digital computers. It is based on

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calculating the digital operations per second equiva-lent to various analog components such as integra-tors, summers, multipliers, etc. The total digitaloperations per second equivalent to typical currenthybrid systems is shown to be upwards of severalhundred million operations per second. This givessuch hybrid computers a one- to three-order ofmagnitude speed advantage over the full spectrumof scientific digital computers.Despite the improvements in hybrid computer

system performance over the past decade, thecomputing load for dynamic system simulation hasshifted more and more to all-digital systems. Thisis because the hybrid advantages of high computingspeed and good man-machine interaction are offsetby the disadvantages of programming complexity,and lengthy problem debugging and turnaroundtimes. In recent years the dramatic cost decreasesand speed increases in digital computers, along withthe development of easy-to-use compilers such assimulation languages, have accelerated the move-ment toward all-digital solution of dynamicproblems.Hence, hybrids have languished, and the general-

purpose hybrid computer market has been in anessentially no-growth posture for a number of years.For this reason the manufacturers of hybrid equip-ment have lacked the resources to develop a new,

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fourth generation of hybrid computer systems thattake full advantage of the present-day componenttechnology which has been so important in thedevelopment of the latest general-purpose digitalcomputers.

In recognition of this, two years ago the U.S.Army Materiel Command let study contracts toeach of the several hybrid manufacturers to deter-mine the optimum hardware and software con-figuration for the next generation of hybrid com-puters. The last two papers in this issue indicatethe future direction of hybrid computer systemsand are in part based on results of the AMC study.One paper is by Rubin and Mawson of EAI and

the other is by Fadden and Graber of AppliedDynamics International. Both papers emphasizethe importance of taking full advantage of currentand projected integrated-circuit component technol-ogy in development of next-generation machines,components such as high-performance IC opera-tional amplifiers, monolithic DAC's and ADC's,low-cost solid-state digital memory chips, and LSIanalog switch arrays. Use of these switch chipswill allow replacement of the hand-patched problemboards on current hybrids with automatic elec-tronic patching. This in turn will permit problemturnaround in milliseconds, and as a result, largehybrid computers will be opened up to many moreusers operating on a time-shared basis throughterminals, both local and remote. The low cost,small size, and low power of the IC componentsin t-he next-generation hybrid will mean that muchlarger hybrid computers can be assembled at costssimilar to today's machines, with correspondingincreases in the cost effectiveness of hybrid com-puters. The availability of electronic patching willalso make more effective the implementation ofhybrid compilers using a simulation language orother source program, so that users with no parti-cular hybrid expertise can easily program thesenew hybrid computers. R

Robert M. Howe is professor and chairman_ of the Department of Aerospace Engineering

at the University of Michigan, Ann Arbor,where he has been involved with teaching

_w g and research in analog and hybrid computers,automatic control, and flight simulation. Hehas been a consultant to numerous com-panies and is a founder of Applied Dy-namics, Inc. (now Applied Dynamics Inter-national).

Howe was the first national chairman of Simulation Councils,Inc. (now SCS), has been a member of several USAF advisorypanels, and has published over 30 scientific articles and onebook. He received his BSEE from Caltech, his AB in physicsfrom Oberlin College, his MS in physics from the Universityof Michigan, and his Ph. D in physics from MIT. He is amember of the IEEE Computer Society, Tau Beta Pi, PhiBeta Kappa, Sigma Xi, AIAA, SCS, and the BiomedicalEngineering Society.

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