A medium scale minicomputer system for laboratory measurementsR. J. Higgins and J. C. Nosler Citation: Review of Scientific Instruments 45, 371 (1974); doi: 10.1063/1.1686632 View online: http://dx.doi.org/10.1063/1.1686632 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/45/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Generation and measurement of laboratory scale focused sonic booms J. Acoust. Soc. Am. 104, 1849 (1998); 10.1121/1.424458 Minicomputer in the Laboratory Am. J. Phys. 52, 1155 (1984); 10.1119/1.13757 Laboratory Minicomputing by J. R. Bourne Med. Phys. 9, 931 (1982); 10.1118/1.595214 Minicomputerbased test system for analyzing the electronic components of a large scale scientific researchinstrument Rev. Sci. Instrum. 52, 915 (1981); 10.1063/1.1136690 A Minicomputer in a Senior Modern Physics Laboratory Am. J. Phys. 39, 1044 (1971); 10.1119/1.1986366

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A medium scale minicomputer system for laboratory

measurements*

R. J. Higginst and J. C. Nosier

Physics Department, University of Oregon, Eugene, Oregon 97403 (Received 23 October 1973; and in final form,S December 1973)

A flexible, powerful, and interactive computer-assisted measurement system is described, based on a 16-bit minicomputer and an extensive set of peripherals. A general purpose hardware-software configuration is described which enables different users to computerize measurements in completely unrelated experiments, with minimal changes in interfaCing. General purpose subroutines to control tile digitizing of measurements are FORTRAN -callable from user programs. The result is a system for computer-assisted measurements which may be used as a general purpose laboratory instrument, requiring only a knowledge of FORTRAN. Illustrative applications are described in several fields of physics. A comparative evaluation of other computerization schemes is given.

I. INTRODUCTION

A. The Medium Scale Minicomputer

Advances in speed, memory, and cost of minicomputers are having a major impact on laboratory measurement techniques. Recently, peripherals such as magnetic disk memory have been developed for minicomputers which make a very sophisticated medium scale computer possible at a cost «$30000.) which will considerably widen the domain of such systems. A medium scale minicomputer-based system has a number of assets in laboratory measurements.

(i) Flexibility: the same hardware can perform many different tasks (as compared with dedicated minicomputer or passive data acquisition system).

(ii) Mass storage of "" 106 words makes possible acquisi­tion and storage of large data files and (by segmenta­tion) execution of programs far larger than the CPU core size.

(iii) Fast interactive graphics can bring physical intuition into computer assisted measurements.

(iv) Computer-assisted measurements can be made in real time, making possible new measurement and analysis capabilities not possible with a time shared data terminal or in the batch mode.

(v) With a disk, program deVelopment is very fast compared to either a paper-tape-only minicomputer or a batch computer. With adequate system software, program compilation, editing, and debugging can go on in a rapid interactive mode, as with a time shared computer.

(vi) Connection of a high speed link to a large scale central computer makes the "number-crunching" assets of a big machine available in the laboratory.

B. The Multiple User Environment

However, the cost of such a fully implemented system is sufficiently high that it is still beyond the reach of most individual investigators. We describe here such a system which has been designed to be shared among several independent investigators, reducing the effective cost to a reasonable level yet retaining the advantages listed above. The basic parameters of what will be called the multiple user environment are as follows.

371 Rev. Sci. Instrum., Vol. 45, No.3, March 1974

(i) Since the users are doing different experiments in different laboratories, and do not share the same instru­ments to gather data, there are potential problems in changing interfaces.

(ii) For the same reason, the user's programs will be all different, yet they may have common computational needs (e.g., digitizing analog signals and graphically viewing results).

(iii) With neither time sharing or batch processing available in this system, there will be only one user at a time.

(iv) There is no full time computer operator; each user will run the machine.

(v) For effective system utilization, down-time between users must be kept at a minimum (a few minutes), even when interface configurations must be changed.

(vi) Few users want to learn either assembly language programming or interfacing.

C. Medium Scale Minicomputer for Multiple User Laboratory Measurements

In the past, most minicomputer applications in the laboratory have been dedicated ones. Here, the classic example is the PDP-8. In such applications, ease of interfac­ing and ease of programming playa secondary role. One generally builds a single interface, and the time invested in logic design and building, testing, and debugging is viewed as part of the start-up time for the system. Similarly, program development in such a dedicated application is done once, and one can tolerate the relatively slow program­ming speed inherent in a paper tape system where the primary input/output may be even as slow as Teletype speed (ten characters per second). The slowness of editing, compiling, and loading programs in such a system leads to a large inertia towards program changes. However, if the application is to involve a variety of measurements using a variety of different pieces of measurement apparatus, ease of interfacing and ease of programming take on a special role. Unless the system chosen is well designed to achieve these goals, the time spent computerizing a given measure­ment might be spent more usefully in making the measure­ments without a computer at all. It is for the second kind

Copyright © 1974 by the American Institute of Physics 371

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372 R. J. Higgins and J. C. Nosier: Minicomputer system 372

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of application that the computer system to be described was designed and therefore we will concentrate on those features which make the system useful to a variety of users with different measurement apparatus.

To achieve the goal of being able to change quickly from one experiment to the next required efficient ways of changing to essential components: the software to drive the data acquisition system, and the hardware interfaces to data acquisition or measurement devices. Achieving software flexibility required the design of a library of subroutines which interact with measurement instruments. For ease in programming, these may be called by a user in a high level language (e.g., FORTRAN), just as subroutines are used to evaluate functions in numerical computati~n. The goal of hardware flexibility was achieved by building a "library" of standard interface cards (most supplied by the manufac­turer). These simply plug in to the computer and connect by cable to a variety of standard devices such as digital voltmeters, frequency counters, and other devices with a BCD output, and to analog inputs. In addition, some more sophisticated applications make use of general purpose digital input/output registers for bidirectional transmission of data.

The hardware is not unique, and in fact its development has been stimulated in part by recent innovations in the use of laboratory computers in other fields (particularly in chemical instrumentationl ). The most important system feature is modular software-hardware "black boxes," requiring a minimum of interconnections to a user's experi­ment, and easy use in a specific user program. The success of this general purpose computer-assisted measurement system is demonstrated in Sec. III by examples of applica­tions made in several field of physics. Very recently, several manufacturers2 of minicomputers have made available an already interfaced I/O package designed for general purpose laboratory measurements. Our system was designed with the same goal, and a report on several years experience with the system may be valuable in evaluating the com­mercial systems now available.

Rev. Sci. Instrum., Vol. 45, No.3, March 1974

4 I I,PLOTTER, I ~ MULTIVERTERI 60 INTERFACE

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II. DESCRIPTION OF SYSTEM

A. Hardware

1. System Configuration

-

DIGITAL EXPERIMENT

CONTROL PROCESSOR

A block diagram of the system is given in Fig. 1. The system centers around a Hewlett-Packard 2116B computer which utilizes a 16-bit word length and 1.6 J,lsec machine cycle time. At present the memory consists of 16 k words, though all of the features described here were developed when the machine had only an 8 k memory. Other fea­tures of the CPU (Central Processing Unit) include the following.

(i) Two channels of direct memory access (DMA). These are used for the communication of input/output (I/O) data to core without the use of an accumulator; with the capability of very fast data rates (up to 500 k word/sec).

(ii) The CPU contains a hardwired integer multiplication and division package which speeds up integer arithmetic by a factor of 100, giving a multiply time of 200 J,lsec.

(iii) An input/output bus which enables connection of peripheral devices through plug-in interface cards. Each I/O channel is assigned a priority for interrupt, allowing the user to establish absolute priority of a device by the location of its interface card.

(iv) An accurate crystal clock which can be activated under program control. The clock is used to time data acquisition from intervals of 100 J,lsec-l00 min.

(v) In addition a variety of dedicated interface cards are used to control the operation of standard peripherals on the system.

2. Disk Operating System

The major peripheral used is a magnetic disk (HP-2870). The disk and the operating system which controls it are the main source of multiuser flexibility.

(i) Users can maintain security of the programs and data for specific experiments by removal of disk cartridge

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373 R. J. Higgins and J. C. Nosier: Minicomputer system

used for that project. This reduces the possibility of acciden­tal erasure by others.3

(ii) Users are able to compile, edit, debug, load, and execute programs under the control of the disk operating system. Since very little slow I/O is required, this results in efficient use of CPU time during programming, compared to paper tape, or even magnetic tape minicomputer systems.

(iii) The disk also becomes the main storage device for data. Files are rapidly transferred to and from the disk with EXEC calls in data acquisition programs.

3. System Console

The system keyboard is a CRT terminal (Computer Terminal Corporation model 3300) which operates at up to 2400 baud (20 times Teletype speed) and is free from the breakdowns common to Teletypes. The combination of the fast CRT terminal and the large disk storage eliminate much of the need for hard copy of transient program during editing and debugging.

4. Hard Copy Output

The system printer is a Centronics lOlA character printer, which has a printing rate of 165 characters per second. This is slow by the standards of .drum printers or the electrostatic printers, but its cost «$4000) is signif­icantly less, and with the use of the fast CRT terminal, most programs can be carried through the DEBUG stage with very few hard copy listings. This decreases the load on the line printer and in that mode the time required for its use is not limiting or wasteful of CPU time.

5. Paper Tape

Paper tape I/O is accomplished with a "fast" photo reader (500 characters/sec), and a "fast" punch (120 characters/sec).

6. Analog Data Input

Two ways are available at present to get analog data into the system: (a) wide band data may go in via a fast A/D (Raytheon multiverter), with 12-bit resolution [±10 V full scale, 8 channels, and 50 kHz sampling rate (max)]; and (b) low frequency high resolution data may be taken via a digital voltmeter and data source interface card (presently this is a HP-3450A DVM (51 digits), though any BCD output DVM may be used).

7. Graphic Display

Graphic display is accomplished in two ways:

(a) For a quick review of data with moderate (8-bit) resolution, an oscilloscope display is used, interfaced by a dual8-bit D/ A. A variety of programs have been developed which make it possible to pan through large data fields, examine small regions, and, utilizing a cursor on the screen, select regions for further analysis or plotting. The 'scope

Rev. Sci. Instrum., Vol. 45, No.3, March 1974

373

programs run in an interrupt mode, with automatic refresh­ing of the display.

(b) Hard copy graphs may be generated on a standard laboratory X - Y recorder. Data generated in the computer are output through a 16-bit duplex register to a special interface. Major design considerations of this plotting system included the following.

(i) Software adaptation-the system can be used under both interrupt and skip-flag modes of output. Plots are generated in either line or point modes in FORTRAN or Assembler language. FORTRAN control commands are similar to those used by the HP-7200 plotter. Subroutine calls such as CALL PLTL (plot a line), CALL PLTP (plot a point), and CALL PLTT (plot terminate) are used to enter the Assembly language driver (100 instructions). A word output to the interface consists of 4 control bits plus 12 bits of data.

(ii) Accuracy-the system employs two lO-bit digital-to­analog converters, giving a resolution corresponding to 0.25 mm on a 25.4X25.4 em plot.

(iii) Speed-there are several speeds at which the inter­face will drive the plotter, depending on the type of plot (points or continuous line) and the distance the pen is to be moved. In the fast line plot mode the plotting speed is 20 data points per second (50 msec per point) at spacing up to 1.27 em. If a move of over 1.27 em (up to 7.6 em) is required the plot rate is slowed and the output filtered so as not to exceed the slew rate on the plotter.

The principal advantage of this design is to make available high quality computer generated plots with a cost of only the interface C'-$lOOO). A standard X - Y recorder is used, which is left available for conventional laboratory applications.

8. Teletype

A standard ASR33 Teletype is also employed as a remote (30 m cable) console at experiments.

B. System Software

The software used by the system falls under three main categories: (i) system programs provided by Hewlett­Packard; (ii) system library routines and programs written at the University of Oregon; and (iii) programs written by the users for their individual experiments.

Hewlett-Packard provided a disk operating system (DOS-M) which provides the main executive functions. The HP system includes an editor, a FORTRAN IV compiler, an assembler, a loader, a cross reference generator, and a standard library package. In addition DOS-M provides a very sophisticated job processor. It is through the execution of the job processor that full use is made of good file manage­ment technique on the disk. All functions such as: editing source files; execution of user programs, the loader or compilers; and file manipulations, are all controlled with simple 2 character mnemonics through the system console.

It is the measurement software which turns the collection of hardware just described into a general purpose computer

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374 R. J. Higgins and J. C. Nosier: Minicomputer system

assisted measurement system which is more than the sum of its parts. Measurement subroutines are written as generally as possible, to be applicable by users with very different measurement needs. The combination of subroutine and measuring instrument becomes a modular "black box," which may be used as a general purpose laboratory instru­ment without knowing the details of what goes on inside the box (how the computer interacts with the instrument). The solftware part of the black box may be duplicated as many times ~s needed, for incorporation into specific user programs as a FORTRAN subroutine.4 Other general programs are designed for interactive oscilloscope graphics or hard copy plotting, and allow users to profit from hands-on control of the computer to analyze, display, and make decisions about data during an experiment.

A brief description of this measurement and graphics software follows. 5 In addition to the general concept above, the programs have two special features. First, the programs (written in HP assembly language) occupy a minimum of core space, as they were written at a time when only 8 k of memory was available. Second, the programs are made to work in the disk operating system environment (and be easily accessible) by being added to the system library, where they are indistinguishable from standard FORTRAN function subroutines.

c. University of Oregon Program Library

1. V oltage Measurement Subroutines

i. High Resolution. DVM is an I/O service function subprogram which samples the HP-34S0 S-digit voltmeter and translates the reading into floating point form. Other parameters generated are an overflow flag and an integer specifying the function setting of the DVM. This high resolution requires a CPU dead time of as much as 66 msec while the DVM is integrating. Program DVM3 makes this time available and utilizes two entry points, enabling the CPU to continue execution during the integration time of the DVM.

ii. High Speed, Low Resolution. ATD is an I/O service function subprogram used to sample a voltage using the 23to Multiverter, and convert the result to floating point form. Because of the slow speed in floating point conversion in the CPU, the maximum data rate with ATD is only 1 kHz. To fully utilize the speed of the Multiverter (SO kHz), program IATD is used. IATD has a minimum software overhead (6 instructions), and returns an integer result with 12-bit resolution in less than 40 J,tsec.

2. Timing

To generate real time idle loops (as in data acquisition at specified rates), two timing subroutines are used. Sub­routine TIMER uses a crystal time base generator to generate delays from to msec to 999 sec. TIMER disables the interrupt system, however, and if this is undesirable (or jf delays of less than 10 msec are desired) then subroutine TIME2 is used. TlME2 does not use the time base generator but rather the time to execute nested DO loops.

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374

3. Graphic Display

i. Oscilloscope Display. Oscilloscope display is used to inspect data array and allow interactive processing (filtering, baseline subtraction, peak location, integration, etc.) prior to further processing. scopl is an I/O subroutine which enables a data array to be continually displayed onto the X - Y oscilloscope. Up to 2000 points are refreshed every 20 msec under interrupted control, so that the CPU is free to continue other tasks. scop3 is an I/O subroutine which displays data onto an X - Y oscilloscope under inter­rupt control. 256 points of an array are displayed con­secutively. A calling parameter supplied by subroutine PAN2 specifies the first word of the array to be displayed, then the next 255 points in the array are displayed from left to right. Panning across a data array is quite simply accomplished by varying this parameter during execution. Another calling parameter specifies the channel (0-255) of a vertical cursor on the screen. During execution the cursor can be steered, via CPU switch register settings, to any location in the array, with cursor location written on the'scope.

ii. Digital Plotting. PLOTI is an I/O service routine used to direct data to be plotted to the plotter interface. Identify­ing information (parameters and labels) may be plotted using subroutines INUM, ANUM, and SYMBL.

4. Debugging, Branching, and Parameter Input

ISWC is a subroutine used to read the CPU's 16-bit switch register under program control. This is often covenient for interactive graphics, or for other parameter input during execution, since the driver for ISWC occupies far less core (5 words) than the system's input/output FORMATTER program (toOO words). HALT is a subroutine used for debug or operator intervention which generates an orderly halt of the computer when executed. A parameter is displayed in the T and A register after the halt, to locate the halt in the program being tested. This makes possible the setting of program parameters and logical branches at execution time without the need for Teletype input.

D. Example of Data Acquisition Program

A few simple examples of how these service routines are used in a high level language are given as illustrations of . their convenience in laboratory measurements. The following FORTRAN statement:

A = FLOAT (ISWC) *ATD (3)/DVM(I,J)

would produce the following chain of events. Upon execution of this statement the switch register would be read in octal and converted to a floating point decimal number; the third channel of the analog-to-digital converter would then be sampled, converted into floating point form and multiplied by the number from the switch register; this number would then be divided by a number generated from sampling the digital voltmeter. The result would then be stored as a variable named A. The execution time of this FORTRAN statement is dependent on not only software but

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375 R. J. Higgins and J. C. Nosier: Minicomputer system

SYSTEM PERIPHERALS USER PERIPHERALS

FREQUENCY AND

TIME

375

FIG. 2. Block diagram showing functional relationship of peripheral devices with the computer in a typical experiment configuration. System peripherals, shown on the left, remain unchanged in all experiments. User peripherals, shown on the right, vary with each experi­ment, but connect through standardized plug-in interface cards. The feedback loop is closed by the user, who can view the results of computerized data acquisition and analysis on graphic and list devices during the experiment. In this interactive mode, the computer is similar to a laboratory instrument, but with more signal processing power and sophistication.

J DIGITAL CONTROL

AND L-__ ...J--<l STATUS

LINE PRINTER XY PLOTTER

also integration time of the DVM plus the sampling time of the ADC.

As another example, suppose it is desired to generate an array from samples of a rapidly varying voltage, and also measure the value of a second voltage accurately at the beginning of the array. Use of program DVM3 makes it possible to encode the DVM, then measure the rapidly varying voltage with the fast A/D, then read the voltage waiting at the DVM'S output register. For example:

CALL ECDUM

DO 10 1= 11000 CALL TIMER (0.001) 10 DATA(1) =ATD(4)

A=RDDVM(I,])

III. APPLICATIONS

This call to DVM3 allows integra­tion of the dual slope DVM to begin and then returns to the next statement. This DO loop samples the 4th channel of the analog to digital converter 1000 times at 1 msec intervals (set by the call to TIMER). This statement stores in floating point form the reading of the DVM as a variable named A (if the integration is not complete when this statement occurs, it will not return to the main program until completed).

That the flexibility described earlier is possible in practice is illustrated by a brief listing of some recent applications. Although designed principally for applications in solid-state and low temperature physics, the system has had application in other fields as well, demonstrating the general applicabil­ity of the hardware and software concepts described above.

A. Solid-State PhYSics

Principal applications have been made to: measurements of magnetization in metals, particularly the de Haas-van Alphen oscillations; the study of electron scattering due to

Rev. Sci. Instrum., Vol. 45, No.3, March 1974

~~~~~~. (ANALOG CONTROLl

impurities6 and dislocations7 ; measurement of magnetic interaction effects,8 absolute amplitudes, and g-factors.9

These measurements have made use of a special interface for multiplexed I/O to provide real time signal processing,1O computer control of many experimental parameters, and interactive operator-computer communication during the experiment. II

B. Low Temperature PhYSics

Study of the dynamics of ions in liquid helium.12 Here, the experiment was located in a nearby laboratory, with connection via cables (approximately 15 m) of high level analog inputs and of both analog and digital outputs for experiment control.

C. Astronomy and Astrophysics

In an experiment on eclipsing binary stars,13 photometric data acquired at a remote observatory were read in on punched paper tape. Although this application could have been done with a conventional central computer (and was eventually transferred to a time shared system), the system's fast paper tape facilities facilitated tape editing, and the system's interactive graphics aided program development. In an experiment in infrared astrophysics,14 interferometric data acquired in a flying observatory was read in from analog magnetic tape using the system's fast A/D, and processed using a Fast Fourier Transform.

D. Atomic Physics

Measurements of intensity profiles of pressure-broadened spectral lines were recorded remotely on punched paper tape. The system was used in interactive graphic processing: digital filtering, peak location, baseline subtraction, and peak parameterization.16

E. User Configurations

A given application is either on-line or off-line, the distinction being whether the user's data requires real time

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376 R. J. Higgins and J. C. Nosier: Minicomputer system 376

TABLE I. Hardware configurations used for typical computercassisted measurements.

Peripheral Off -line user On -lin e user On-line system control

Cartridge disk: User disk System disk

System console (CRT terminal)

Line printer Teletype

Program storage (especially, segmented long programs) Data file storage (as acquired and after processing) Disk monitor, executive, library, compiler, assembler, editor. Working file storage Control of monitor in compiling, editing, loading, executing programs; Rapid viewing and updating of program files and data files Rapid hard copy of program listing. Printout of data acquired and/or processed.

Remote status (distant experiment) Preparation (off-line) of program tapes. Interactive graphic processing of data. . Oscilloscope

x-v plotter Hard copy graphs of processed data.

Interactive tuning up of system parameters while viewing real time display.

Graphing processed data during experiment. Optimize experiment output by seeing results during experiment.

Paper tape reader

Paper tape punch

Input source programs for compile and edit. Input off-line data. Fast dump of machine-readable output: program listings (edited version, or backup of disk copy); executable binary

programs for paper-tape-only minicomputer. BCD output from measurement instruments such as DVM and frequency

counter. Data source interface(s)

Analog-digital converter

Digital-to-analog

Digitizing analog magnetic tape Fast (30 kHz) medium resolution (12-bit) digitizing of analog signals records multiplexed to 8 channels.

converter(s) Relay register Digital experiment

control processor

CPU access during the experiment. Switching from one user to another ordinarily requires no more than exchanging user cartridge disks, and changing a few cable connectors between the data interfaces and a specific user's measuring instruments. Under these conditions, a down time of less than ten minutes is required between quite different experimental configurations. A schematic view of a con­figuration is shown in Fig. 2. The pattern of use of the various peripherals in different experiments is shown in Table I.

IV. EVALUATION OF COMPUTER·ASSISTED MEASUREMENT SCHEMES

A. Evaluation: Medium Scale Laboratory Computer in a Multiple User Environment

The original design of the system was described in detail in the Introduction. The initial design was successful in minimizing the time required to get the computer system running. The system, as delivered, was "on the air" the first day it was turned on. The use of "off the shelf" inter­faces and adequate and reliable system software made it possible to very quickly get to the intended business of laboratory measurements, rather than getting involved in computing hardware or software.

With experience, however, the original design proved somewhat restrictive. For example, when a plotter was needed, an interface was designed and built to drive a standard X - Y recorder. Later, a quite sophisticated special interface was built for real time multiplexed experiment control.ll In such cases, however, a standard interface card (with slight modification) serves as a buffer register, simpli­fying the interface design, and protecting the CPU.

Rev. Sci. Instrum., Vol. 45, No.3, March 1974

Control of experiment via analog feedback signals. Driving power-supplies, servos, etc.

Control of instruments via switch closures. Multiples many I/O functions in

single line. Experiment control via digital or analog methods. Experimenter-computer inter­action.

Similarly, it proved unnecessarily restrictive to avoid assembly language programming. The program library described earlier was developed locally, with contribu­tion from several users, and stimulation from evolving applications.

The extensive array of peripherals shown in Figs. 1 and 2 has evolved to meet a variety of needs (e.g., the CRT terminal and line printer remove an earlier I/O bottleneck, the Teletype). Sharing the system makes such improve­ments more feasible. Sharing the computer among users in different laboratories is not without its problems. The principal problem has been scheduling. In busy periods, 24 h/day usage is common, and a priority scheme has evolved. In decreasing priority, users sign up for blocks of time for system maintenance, data acquisition (real time), data reduction, and program development.

B. Evaluation: Other Alternatives

i. Lab Computing vs Central Computing

The medium scale laboratory computer, oriented towards measurements and data acquisition, complements the capabilities of central computing facilities, oriented towards data processing, by providing an array of simple ways to put analog signals into machine readable form. In our system, this is exemplified by AID peripherals used directl y and also indirectly (e.g., reading analog magnetic tape records). In another application, a very inexpensive coor­dinate digitizer was constructed to digitize graphical information, simply by linking X and Y coordinates to potentiometers read by two channels of the A/D converter. The lab computer has the peripherals and the software oriented towards measurements, and results in such

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377 R. J. Higgins and J. C. Nosier: Minicomputer system

laboratory tasks being accomplished with a modest invest­ment of time and hardware. The central computer, on the other hand, retains its superiority in processing and storing large masses of data. The two are complementary, rather than competitive and it is therefore natural to envisage tying them together in a hierarchy or network; such projects are underway in many laboratories. l

2. Time Sharing in Data Acquisition

The use of a central time-shared system has certain applications in laboratory data acquisition,14 but only if the time domain of the measurement is sufficiently slow. In evaluating a time shared connection for measurements, several factors enter: (i) data transmission rate, ranges from 110 baud (Teletype speed) to a factor of 100 faster with special high speed links; (ii) access time to the monitor and uninterrupted connect-time when transmitting data; (iii) reliability: what if the system is down when the measure­ment is up or what if the system crashes partway during transmitting an array of data? The first two items reflect the difference between human and machine data sources. A time shared system which is fast enough for humans typing may be easily swamped with laboratory data sources. Fast links to time-shared systems are becoming available, but general experience to date strongly suggests the need of a local buffer with stand-alone data acquisition capability. A minicomputer, a "smart" terminal, a calculator-based data acquisition system, or a medium scale computer such as the system described here are natural data links to a time shared system.

3. Applications to the Dedicated Minicomputer

As discussed in the introduction, a paper-tape-only minicomputer suffers from a serious I/O bottleneck which is particularly frustrating during program development. Although compilers and simulators may be available which will run on a large central computer, it is not possible to realistically simulate execution of (hence debug) programs in which the computer is interfaced with laboratory measure­ments. A medium scale system, however, can assist speedy program development for a dedicated computer, if the CPU's share the same assembly language and hardware interface structure. There is therefore a considerable cost and efficiency advantage in selecting a dedicated mini­computer from within the same CPU family as the mini-

Rev. Sci. Instrum., Vol. 45, No.3, March 1974

377

computer used in the medium scale system, since program compilation and debugging can be done- using the powerful peripherals of the medium scale system, which then produces an absolute paper tape for the dedicated minicomputer.

ACKNOWLEDGMENTS

The basic computer system was made possible by a National Science Foundation Science Development Grant to the University of Oregon. System expansion was made possible by a grant from the NSF's Office of Computing Activities. We wish to thank H. G. Alles for important contributions in system hardware and software development. Professor R. J. Donnelly and Professor D. H. Lowndes, as members of the User's Group, have assisted with and stimulated many facets of the system's development. We are grateful to several individuals whose advice on mini­computers aided the planning phase of this project. These include Professor C. Klopfenstein, Professor H. W. Lefevre, Professor R. Haller, and Professor G. Struble. This manu­script was written while one of the authors (RJH) was a resident visitor at the Bell Laboratories, Murray Hill, New Jersey; we would like to thank that institution for its hospitality.

*Research supported by the National Science Foundation. tResident visitor: 1972-73, ~ell Laboratories, Murray Hill, N. J. IFor example, C. Klopfenstem, J. Chromatograph. Sci. 10, 22 (1972). 2For example, the Digital Equipment Corporation's Laboratory

Peripheral System for the PDP-11, and Varian's ADAPTS for the 620 series.

3Although the system area of the disk is protected with H-P's DOS-M disk operating system, the user area is not protected from being accidentally written over or erased.

'It is this feature which results in most users being able to avoid learning Assembly language and interfacing.

-The programs are described more completely in a document, "University of Oregon-H-P System Library," available from the authors upon request.

6H. G. Alles, R. J. Higgins, and D. H. Lowndes, Phys. Rev. Lett. 30, 705 (1973); H. G. Alles and R. J. Higgins, Phys. Rev. B (to be published).

7Y. K. Chang and R. J. Higgins (unpublished). 8H. G. Alles and D. H. Lowndes, J. Phys. E6, 895 (1973). Phys.

Rev. B (to be published). 9H. G. Alles, R. J. Higgins, and D. H. Lowndes (unpublished).

10H. G. Alles and R. J. Higgins, Rev. Sci. Instrum. (to be published). llH. G. Alles and R. J. Higgins (unpublished). 12D. K. Cheng, M. W. Cromar, and R. J. Donnelly, Phys. Rev. Lett.

31, 433 (1973). 13E. Ebbighausen (unpublished). 141. G. Nolt, J. V. Radostitz, and R. J. Donnelly, Nature 236, 444

(1972). I-D. Gilbert and S. Y. Ch'en (unpublished). 16D. Giovanielli, J. Merrill, and A. Luehrmann, Rev. Sci. Instrum.

41, 1187 (1970).

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