IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS AND CONTROL INSTRUMENTATION, VOL. IECI-18, NO. 2, MAY 1971

Minicomputers and Peripheral Equipment-A Current Review

ERNEST T. ROLAND, MEMBER, IEEE

Abstract-The cost-performance characteristics of minicomputersand peripheral equipment from the systems engineer's point of vieware reviewed. The minicomputer review covers essential differencesin memory unit, CPU, I/O structure, and software for the widevariety of machines on the market. The peripheral review coverscost-performance for various l/O devices and emphasizes that pe-ripheral costs may greatly exceed the basic minicomputer costs.

Process control instrumentation accuracies are presented and thefundamental guidelines for implementing an optimum system arediscussed.

INTRODUCTIONI N the past year the major developments in the mini-

computer field, as they affect process control, canbe summarized as follows.

1) There are still new minicomputers being broughtinto the market. New manufacturers are making thescene, and established manufacturers are bringing newmodels. In general, there are more options available.

2) The peripheral market is ebullient, with a greatdeal of truly low-cost peripherals reaching the market.

3) The number of process control installations utiliz-ing the minicomputer has increased dramatically. Con-tinued growth is also the general prediction (a predictedfive-fold dollar volume increase in the next five years).In the foreseeable future, minicomputers will be an in-tegral part of the control scheme; digital systems willoccupy a position of importance in process control sim-ilar to the one now enjoyed by EDP in the business en-vironment.

MINICOMPUTERS AND CONTROLLERS

The literature abounds with information on mini-computers. At a recent count there were 60 differentmodels to choose from.No attempt will be made to enlarge on this great

abundance of specifications. The computer-controllerwill be reviewed primarily as a system element.As a result of competition, there is still a downward

trend in prices. Typical low-price entries are the Vari-systems PAC-16 and the forthcoming Computer De-velopment, Inc. CD-2000. Many established manu-facturers have within the past year reduced prices ontheir standard lines, some as much as 20 percent.As a rough approximation, the elements of cost on a

typical minicomputer selling for less than $10 000 areshown in Fig. 1.

Manuscript received August 26, 1970. This paper was presentedat WESCON, Los Angeles, Calif., August 25-28, 1970.

The author is with Jackson Associates, Columbus, Ohio 43220.

LINE PRINTER

PERIPHERALS

Fig. 1. Appropriate elements of cost in a mini-computer selling for less than $10 000.

The direct cost comparison between different ma-chines should include a clear understanding of whatoptions, if any, are included in the price. However, priceis only meaningful if related to basic machine char-acteristics as compared to the final system application.Therefore, the system evaluation would require investi-gation ofmemory unit;central processing unit (CPU);I/O structure;software.

Ml1femory UnitAn important characteristic as it relates to cost is the

memory size, as specified by the number of words andthe word length. The typical core memory capacity is4096 words, although there are machines such as thePAC-16 offered with as little as 1K core and special ap-plications with certain machines may eliminate all coreand use read-only storage.

Minicomputers are available with 8-, 12-, 16-, and18-bit word lengths. Presently the most common designchoice is 16 bit, and this trend is evident in the DigitalEquipment Corporation product line; the well-knownPDP-8 family is a 12-bit system, and the newer PDP-1 1is a 16-bit system (there are also other fundamentalarchitectural differences between these machines).Eight-bit machines which usually have variable wordlength and microprogram control also are popular inthe low-price range.

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ROLAND: MINICOMPUTERS AND PERIPHERAL EQUIPMENT

An important system engineering consideration is thecore increment size and the maximum expansion. Themore common value is 4K for the incremental size. Theexpansion capability is usually up to 32K words; how-ever, there are machines which are designed for expan-sion up to 256K words.A characteristic which is often advertised is memory

cycle time. Presently memory cycle times range from600 ns to 8 pus, with values around 1 us being typical.Technological advances resulted in substantial decreasein cycle times; ferrite cores still dominate the field;however, MOS and other techniques offer possibilitiesof future lower costs and perhaps faster speeds. A wordof caution is that the importance of cycle time can onlybe properly evaluated in conjunction with the instruc-tion set, since some instruction sets may be more ef-ficient than others for the same job. A final criterion inestablishing cycle time requirements is the system re-quirements, both present and future. The concept ofdata rate is a useful one in this respect, and the systemthat satisfies this requirement at the lowest cost is theoptimum choice.

Usual options offered with the memory unit are paritycheck and memory protection.

CPUThe instruction set is a key characteristic of the cen-

tral processor. It was pointed out that for a given tasksome instruction sets may be more efficient than others.The answer relates to the machine organization; in thisrespect the parallel-binary organization is prevalent.

In this type of organization, most frequently used in-structions are quite universal. An example is the "shift"instruction which is a simple register operation instruc-tion. The execution of this type of instruction requiresscarcely more than the memory cycle itself. Other ex-amples, each taking two memory cycles, are the "load"and "store" class of instructions; these require one in-struction fetch and one data reference cycle.

It should be noted, however, that some parallel-binary machines do not follow the usual pattern. In thedesign of model 3, Interdata deliberately sacrificedprocessing speed for the flexibility in microprogrammingand the use of read-only memory. The Interdata 4 and5 use different compromises between speed and flexibil-ity and come closer to the standard pattern.

Further, it should be emphasized that not all com-puters have parallel-binary organizations. The PDP-8/S is an example of a machine organized in serial-binary fashion. In this case the decrease in processorhardware is traded off against increased processingtime.

In some instances, the organization is an interestingmix between serial and parallel characteristics. Thesemachines have a parallel-binary organization at thebyte level; however, multiple byte words are processedserially by byte. This organization frees the programmerto define any word length he chooses in multiples of 8

bits. In some applications, the advantage of increasedword length at the programmer's option far outweighsthe disadvantage of increased processing time due tothe serial operation. A typical example of this type oforganization is the line of minicomputers offered byBusiness Information Technology, Inc.The Micro 800 series offered by Microsystems, Inc.,

makes extensive use of file registers which can be usedin essentially any desired manner under micropro-gramming control. Microprogramming (firmware) addsa degree of flexibility over fixed instructions which maybe a desirable feature in certain applications.As inferred in the preceding discussion, the number

of general-purpose registers and memory index registersare an important CPU characteristic. Other charac-teristics are the number of words directly addressableand the type of addressing. Finally, a characteristicwhich is often quoted is the time required for multiply-divide operations. Again, any one of these characteris-tics must be fully weighed against the specific systemrequirements.

I/OThe I/O capabilities of minicomputers vary to a

large degree. Most have optional provisions for one ormore direct memory access channels through whichhigh-speed peripheral devices such as magnetic tapes,drums, or disks can communicate directly with corememory on a cycle-stealing basis.A typical minicomputer can handle normal process

control functions through its "party-line" I/O system,using its interrupt capability to service each of up to 64devices as they request attention. Some of these I/Odevices will operate in BCD or ASCII codes. The sub-routines to perform the code conversions of numbers toand from the internal binary format will require about215 and 260 ,us, respectively, for four decimal digits.The time required for the interrupt subroutines to

handle a single input or output is difficult to specify ac-curately. This is because of the different machine or-ganizations and because of the variability of the inter-rupt handling. Even on single machines, optimummethods of determining which device needs service willdepend upon the number and speeds of the I/O devicesconnected. One might estimate an average time for thetypical minicomputer to perform a single I/O operationin a priority interrupt subroutine between 50 to 100 As.Thus data rates up to 10 kHz or higher can be dividedas needed among the control functions operated in theinterrupt mode of this system.

SoftwareSoftware packages available from the manufacturer

or systems houses have increased greatly in the pastyear. Yet, it is seldom that an existing package will fitexactly the user's requirements, and adaptions are away of life.

55

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS AND CONTROL INSTRUMENTATION, MAY 1971

TABLE I

Number of Supplier-s

Minicomputers over 60Incremental magnetic tape recorder 29Continuous magnetic tape recorder 49CRT displays 57Printers 68X-Y plotters and drafting machines 20Disks 44Drums 13

7.

6-J:<t 5-J0 40

I-)

...... ..

012345 10 15 20SPEED (CHARACTERS/SECOND)

Another trend has been the introduction of compilersfor higher level languages, such as Fortran, Basic,Algol, etc. Although the subject is controversial, theauthor feels that in most cases the disadvantages ofhigher costs and slower running speeds far outweigh any

possible conveniences. The minicomputer in process con-

trol applications loses much of its justification if it ishandicapped by the heavy burden of a compiler. In-stead, for those situations where the user insists on a

higher level language as a matter of convenience, theauthor believes that an executive with the capability ofcall-out in this language is a better compromise.

Higher level languages created especially for process

control, such as Indac, have also appeared. A majorequipment manufacturer behind any one of them iscertain to impart market acceptance. Again, the oftencited justification is user's convenience, but this must becarefully balanced against optimum performance andcosts.

Peripherals

New peripherals specially developed for minicom-puters are reaching the market in increasing numbers.Peripheral equipment is the segment of the computermarket predicted to show the greatest growth rate, andmany manufacturers have geared for it. Consequently,the systems engineer has a wide choice.

Several manufacturers now offer tape decks and cas-

sette recorders around $3000; disks have broken the$10 000 barrier. Inexpensive line printers, some startingat or as low as $3000 are available. CRT displays at lessthan $3000 are offered from several manufacturers.Table I shows the number of companies supplying dif-ferent peripherals; it is not to be construed as all in-clusive but simply to illustrate the degree of competi-tion.

Figs. 2 through 5 deal with cost comparisons. Fig. 2is a plot of the cost range for the various output devicesas a function of speed. The obvious conclusion is thatone must be prepared to justify the costs of increasedoutput speeds. Fig. 3 illustrates the costs per bit ofvarious storage devices as a function of access time.Fig. 4 gives cost as a function of throughput and ac-

curacy for an eight-channel multiplexor; although thisdate is for one particular manufacturer, it clearly in-

dicates the cost trends. For this same manufacturer the

Fig. 2. Cost of terminals versus speed.

10

z

0.1

<)0.01

IUS 10 US 100 US rMS IOSM IOOMS SEC

ACCESS TIME

Fig. 3. Cost per bit as access time.

C,)cn

-J

0

F-(n0

30 40 50 60 70 80 90 100 KHZ

Fig. 4. Cost versus throughput 8-channelmultiplexor and A/D converter.

30 -C,)

< 2.5.-j

0

2.0.0

H: 1.0

0.0

cn

00.

0.5.

12 BIT

1BIT

0 2 3 4 5 6 7 8 9 10 11.12

NUMBER OF CHANNELS

Fig. 5. Cost versus number of channelsD/A converter ± 10-V output.

25 - (X 10)

56

MAXMIN

ROLAND: MINICOMPUTERS AND PERIPHERAL EQUIPMENT

cost as a function of the number of channels and ac-curacy for D/A converters is shown in Fig. 5. The pointis that minimizing systems costs will require use ofequipment which is chosen to match the overall systemaccuracies.

It is also a foregone conclusion that, depending on thesystem complexity, the cost of the other hardware itemsmay greatly exceed the minicomputer costs. In this re-spect, the simplest minicomputer system would consistsolely of a teletypewriter, which adds about 10 percentto the minicomputer cost; in a more complex process,control hardware costs could easily be three or fourtimes the minicomputer cost.Another point to be considered by the systems en-

gineer is the question of interfaces. With the ever in-creasing number of minicomputers and peripherals, inmany situations the systems engineer may be faced withunavailability of interfaces from the hardware manu-facturers. Although they can be easily designed, thesystems engineer must have this cost factor in mind.

Finally, the absence of industry-established standardswhich are widely accepted requires careful investigationof compatibility.

Selection for Process ControlThe question is, then, what to choose. Minicomputer

systems in process control fall in one of the followingclassifications:

data acquisition;direct digital control;combination of the two above;hierarchy member;dedicated special-purpose machine.

In all these applications, the sensors are the govern-ing interface with the process. A quick look at Fig. 6shows that the 16-bit machine is more than adequate;the exceptional application excepted. Generally expectedaccuracy derived from [1].The first job is to match the capabilities of the mini-

computers with the requirements of the process controlsystem.One key step is to analyze the relative importance to

the control system of the various phases such as dataacquisition, set-point control, calculation and data re-duction alarm reaction, and report generation. Once therelative importance of these steps is tabulated, the se-lection process becomes simpler. If the control systemmust react quickly to alarm conditions to preventcatastrophic loss, the field of choice must be restrictedto those minicomputers with fast hardware priorityinterrupt features. If, on the other hand, report gen-eration and data communication to and from character-oriented devices such as readers and printers are im-portant requirements, a machine with good byte-manipulating features is required. The relative need

5.C

1.0

0

ErI

D-

.5

'1

.05

SOLIDSLEVEL , - MOISTURE CONTENT OF SOLIDS

HUMIDITY OF GASES

ELECTRICALQUANTITIES

\1- OPTICAL PROPERTIES

_ < FLUID FLOW FLUID DENSITY ANDFLUIDFLOW SPECIFIC GRAVITY

VISCOSITY \AND CON- \

_SISTENCY_ l\ PH CONCENTRATION 8 OXIDATION POTENTIAL

_ COMPOSITION BY SPECTROSCOPYCOMPOSITION BY OTHER PHYSICAL PROPERTIES

COMPOSITION BY CHEMICAL PROPERTIES

TEMPERATURE| l\~LIQUID LEVEL

PRESSURE_VELOC ITY |l

ELECTRICALCONDUCTIVITY

WEIGHT & WEIGHTRATE OF FLOW

2 4 6 8 10 12 ACCURACYN NO. OF BITS)

Fig. 6. Process instrument element accuracy.

for high-speed I/O and fast fixed or floating pointarithmetic or for nearly perfect reliability such as givenby read-only memory will further restrict the selectionrange. The ultimate criterion is, of course, the cost.

SUMMARY

The number of minicomputers in process control in-stallations has increased dramatically. At the sametime, different minicomputers and peripherals are be-coming available in increasingly large numbers. Thetask of the systems engineer is to match his require-ments against the options available. In doing so, heevaluates the minicomputer in terms of

memory unit

CPU

I/O

software

word length, minimum number ofwords, cycle time, core increment size,maximum expansion, and other op-tions;instruction set, number of general-purpose registers and memory indexregisters, type of addressing, etc.;interfacing requirements, DMA, in-terrupts, etc.;degree of availability.

In choosing the peripheral equipment, systems con-siderations relate to speed, accuracy requirements, in-terface requirements, and so on. The important factoris that in many applications other hardware costs may

57

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS AND CONTROL INSTRUMENTATION, VOL. IECI-18, NO. 2, MAY 1971

greatly exceed the basic minicomputer costs. There-fore, careful systems engineering must be undertaken.Since software may represent also a significant con-tribution to the total systems costs, the proper balancebetween software and hardware must be achieved.

For a given set of system specifications many com-patible combinations of minicomputer and peripheralequipment exist. It is the designer's goal to achieve

maximum reliability and minimum cost within theframework of the system operating requirements. Thusthe selection of that unique computer-peripheral setwhich provides optimum results is the system designer'sjob.

REFERENCE[11 'Chemical engineering 1969 guide to process instrument ele-

ments," Chem. Eng., June 2, 1969.

Discussion of "Minicomputers and PeripheralEquipment-A Current Review"

HUBERT G. DEVRIES, MEMBER, IEEE

Abstract-This discussion of "Minicomputers and PeripheralEquipment-A Current Review" was prepared for the panel pre-

sentation and discussion held atWESCON in August 1970.

Cn ERTAINLY, one of the most important points tonote in Mr. Roland's paper' is that even thoughthere are over 60 minicomputer manufacturers,

new manufacturers are still making the scene. The totalminicomputer market for 1975 is estimated at approx-

imately $600 million. Five manufacturers are expectedto dominate all but approximately $250 million of thismarket. The $250 million divided among the remaining55+ suppliers would suggest an average company sizein 1975 of approximately $5 million annual sales. Thissize company potential relative to the investment andother market factors involved should cause the numberof manufacturers to increase for some time.The new peripherals especially developed for mini-

computers is an interesting area. Certainly the lack oflow-cost peripherals has held up the expansion of theminicomputer market. Probably one of the most neededdevices at this time is a low-cost tape recorder such as

the tape cassette. Next is a low-cost hard copy device.Low-ccst keyboards and display devices are probablynext. Mr. Roland's Figs. 2-5, provide some excellent in-sight into the present types of peripherals, and theircost versus performance criteria.

Another interesting point to note is that the applica-tions are beginning to strongly influence the computer

Manuscript received August 26, 1970. This paper was presentedat WESCON, Los Angeles, Calif., August 25-28, 1970.

The author was with the Systems Division, Ferroxcube Corpora-tion, Englewood, Colo. He is now with Monolithic Systems Corpora-tion, Englewood, Colo.

1 This issue, pp. 54-58.

design. The instruction set, word length, memory size,input/output, software, peripherals, and memory areall being challenged by the user. This will call for muchmore flexibility from the manufacturer's point of view.The number of suppliers of minicomputers will alsoforce this flexibility even from the largest manufac-turers. This demand for flexibility also comes from moretechnically astute users and the fact that a large num-ber of minicomputers will be resold as a part of a largerpiece of equipment.

Mr. Roland's comments on software are very interest-ing. The running efficiency and storage limitations onminicomputers almost dictates that the most efficientlevel of programming be used. Generally, this is ob-tained by programming in assembly language. Cer-tainly, an excellent assembler is one of the most im-portant parts of the successful minicomputer. At presentit appears that the higher level languages are being usedmostly as benchmarks in performance and maturity ofthe machine rather than actual necessary requirementsof a successful minicomputer.The term "controller" is now a well-accepted term

and is used in many cases to mean the same as "mini-computer." At Ferroxcube the term "digital controller"is defined as a machine which resides between a memorysystem and the minicomputer and is cost and perform-ance compatible with this hardware position. Fer-roxcube's Digital Controller FDC-300 will provide theuser with another choice of cost/performance tradeoff.

Mr. Roland's current review provides us with asummary of data on minicomputers from which we candraw conclusions and predictions of our own. This is,no doubt, the most important part of this review, sincein a rapidly developing market such as the minicom-puter/controller, conclusions are quickly outdated.

58