On-Line Data-Acquisition Systems in Nuclear Physics, 1969 — A Reader’s Guide

_On-Line Data-Acquisition Systems in Nuclear Physics, 1969_

Ad Hoc Panel on On-Line Computers in Nuclear Research Committee on Nuclear Science National Research Council

_NATIONAL ACADEMY OF SCIENCES Washington, D.C. 1970_

This is a report of work under Contract NSF-C310, T.O. 47 between the National Science Foundation and the National Academy of Sciences and under Contract AT(49-1)3236 between the U.S. Atomic Energy Commission and the National Academy of Sciences.

Committee on Nuclear Science 2101 Constitution Avenue Washington, D.C. 20418

The first digital electronic device employed to collect nuclear data was the binary electronic counter (scaler) of the 1930's. In the next decade single and multichannel pulse-height analyzers appeared, still using vacuum tubes. In the 1950's the development of multichannel analyzers continued vigorously, with vast improvement of the analog-to-digital converter sections and with the introduction of computer-type memories, based first on acoustic delay lines and a short time later on ferrite cores. The replacement of vacuum tubes by transistors beginning in the latter half of the 1950's accelerated the pace of development and application of all types of electronic laboratory instruments.

The 1960's was the decade of the computer. Before the 1960's almost no on-line computers were used in nuclear research, but since about 1962 the computer has moved into the nuclear laboratory. It provides the research worker with an immensely flexible, powerful, and accurate tool capable of raising the research output of a laboratory while eliminating the most tedious part of the experimental work.

The phenomenal speed of development of computer hardware, software, and methodology contributes to the difficulty experienced by everybody involved in decision-making processes regarding data-acquisition systems. Since the cost of a computer system is often a sizable fraction of the total cost of a new laboratory, there is urgent need for a set of guiding rules or principles for use by a laboratory director planning a system, a reviewer going over a proposal for support, or a potential funding agency considering proposals and reviews. The purpose of this report is to assist in filling this need. The material presented is current through 1969. Although we deal with a field that is developing rapidly, we hope that a substantial portion of the material covered will have long-lasting value.

The report was prepared by the Ad Hoc Panel on On-Line Computers in Nuclear Research of the Committee on Nuclear Science, National Research Council. Appointed in March 1968, the Panel first met in Washington, D.C., on April 22, 1968.

The original members of the Panel were H. W. Fulbright, H. L. Gelernter, L. J. Lidofsky, D. Ophir (through late 1968), L. B. Robinson, and M. W. Sachs. In June 1968, this group prepared an interim report. L. J. Lidofsky was on sabbatical leave in Europe and therefore could not participate during the academic year 1968-1969. Early in 1969 J. F. Mollenauer and J. Hahn joined the Panel.

The Panel has reviewed the present state of the field and has attempted to anticipate future needs. We have agreed on many important matters, including especially useful design features for computers employed in data acquisition, as well as types of organization of data-acquisition systems suitable for various purposes, types of software that manufacturers should supply, and approximate costs of systems, and we present a number of recommendations in these areas. However, the Panel makes no recommendation on standards for computer hardware, such as logic levels and polarities, because of a conviction that these are now rapidly being established as a result of sound engineering progress and the pressure of economic competition in the fast-moving computer business.

Throughout this report we have expressed opinions based on our own experience and on the best information at our disposal. The nature of the report seemed to demand some discussion of properties of specific computers by name. We have tried to be neither misleading nor unjust in our evaluations.

We wish to thank everyone who has aided us, especially P. W. McDaniel, C. V. Smith, and G. Rogosa of the U.S. Atomic Energy Commission and the many scientists in AEC-and NSF-sponsored laboratories who supplied the basic data on which the economic survey chapter is based. We are indebted to several members of the staff of the Department of Physics and Astronomy of the University of Rochester for assistance in the preparation of the manuscript, especially Mrs. Brignall and Mrs. Hughes. We also received initial directions and many helpful suggestions from D. A. Bromley, Chairman of the Committee on Nuclear Science, F. S. Goulding, Chairman of the Subcommittee on Instrumentation and Methods, W. S. Rodney and P. Donovan of the National Science Foundation, and Charles K. Reed, Executive Secretary of the Committee on Nuclear Science.

H. W. Fulbright, _Chairman_ H. L. Gelernter J. F. Mollenauer J. Hahn L. B. Robinson L. J. Lidofsky M. W. Sachs

1. THE TASKS AND THE COMPUTER 1 A. Introduction 1 B. The Tasks 2 C. The Computers 3 D. Matching Computers to Tasks 5 E. On Characteristic Features of Computers and Related Equipment 6

2. DATA-ACQUISITION SYSTEMS 16 A. Introduction 16 B. A Small Time-Shared Data-Acquisition System Based on a PDP-7 Computer 19 C. A Small System Based on a PDP-8 Computer 23 D. A Medium-Sized On-Line Computer System 28 E. A Large System Based on a Single Computer (The Yale-IBM Nuclear-Data-Acquisition System) 32 F. Multiple-Computer Systems 39 G. A Process-Control System: The Brookhaven Multiple Spectrometer Control System 48 H. Relationship to a Remote Computing Center 54

3. A REVIEW AND ANALYSIS OF EXPENDITURES 58 A. The Nature of the Data 58 B. Breakdown of Data for Analysis 59 C. Types of Computers 61 D. Some Total Costs 61 E. Breakdown of Costs by Systems 64 F. Rotating Memory Devices 65 G. Systems On-Line with Computing Centers 65 H. Anticipated Future Expenditures 65 I. Investment in Accelerators, Computer Systems, and Laboratory Budgets 66 J. Process-Control Application 67

4. SUMMARY AND RECOMMENDATIONS ON SYSTEM PLANNING 68 A. The Need for On-Line Computer Systems 68 B. Where Should Large-Scale Calculations Be Done? 69 C. Exercising Economic Judgment in Planning 70 D. On the Utility of Small and Medium-Sized Computers 71 E. Growth Considerations 71 F. Short Summary of Conclusions Regarding System Planning 73

Appendix A: TABLES OF PROPERTIES OF SMALL AND MEDIUM-SIZED COMPUTERS 79

Appendix B: BACKGROUND INFORMATION FOR CHAPTER 3, A REVIEW AND ANALYSIS OF EXPENDITURES 86

THE TASKS AND THE COMPUTERS

On-line data-acquisition computer systems are made in a wide range of types and sizes. In all cases at least one electronic computer is involved--a stored-program machine--because wired-program devices such as pulse-height analyzers are not considered to be computers. The rest of the system typically consists of input/output (I/O) devices such as analog-to-digital converters (ADC's), printers, cathode-ray oscilloscopes, plotters, and control devices, which may include, in addition to the console typewriter, switch boxes to simplify the control of special types of operations and perhaps a set of logic circuits associated with the input system, used to provide preliminary selection of incoming data. In a small but increasing number of cases a computer is seen dedicated entirely to a "process-control" application such as the automatic adjustment of the shim coils of a variable-energy cyclotron or the control of data acquisition in a nuclear-scattering experiment, adjustments such as changing the angle of observation being made essentially under direct automatic control of the computer. The smallest on-line systems use the smallest commercially available computers; the largest use computers bigger than those which until recently served most computing centers. Large systems sometimes include one or more satellite computers. The cost of individual systems ranges from $25,000 to $1,000,000, approximately. The total cost of computer systems in low-energy nuclear laboratories is estimated by now to have reached about $20,000,000. (There has been a larger expenditure in the high-energy nuclear field, where computer systems have been employed extensively for some years longer and where experiments are so expensive that the economic advantages of computer use were quickly recognized.)

We first list the main uses to which on-line computer systems have been put. We start with the simple operations, which we call Class 1.

_Class 1 operations_:

a. Accepting digital data from external devices and storing it in computer memory.

b. Preliminary processing of incoming data, on-line, before storage. This usually involves only operations of logic and simple arithmetic.

c. Controlling the presentation of data via cathode-ray oscilloscope or typewriter, often for the purpose of monitoring the progress of an experiment.

d. Controlling the recording of digital data on magnetic tape, paper tape, or other storage medium.

e. Controlling an incremental plotter.

f. Controlling the output of large quantities of data via a line printer.

g. Transmission of quantities of data between two computers or between a computer and a pulse-height analyzer or other device having a magnetic core memory.

Several operations of intermediate complexity we will label Class 2.

_Class 2 operations_:

a. Processing of data already accumulated and stored either in memory or on tape or other medium (off-line processing). This data reduction is often more complicated and lengthy than the preliminary on-line processing referred to in (Class 1b).

b. Calculation of information required by the experimenter during the experiment, for example, kinematics tables and particle energies corresponding to field strengths in analyzer magnets.

c. Process-control operations, in which the computer directs or regulates a sequence of events in an experiment. Under program control the computer monitors the course of the experiment and supplies signals that cause automatic changes in experimental conditions, such as starting and stopping times of event counting, angles of observation of scattered particles, and accelerator energies. Such applications are designed to relieve the experimenter of unnecessary labor and to reduce the probability of error in routine operations.

Our final class involves even more complex calculations.

_Class 3 operations_:

a. Complicated treatment of reduced data, including least squares and curve fitting.

b. Large-scale calculations such as those required for the evaluation of theoretical nuclear scattering and reaction cross sections, e.g., DWBA calculations, which may each require running times of the order of minutes, even at a modern computing center.

Apparently Class 3 operations do not always have to be done during the course of the experiment; in fact, they can in most cases be carried out later, leisurely, at the local computing center. Nonetheless, calculations of the first type, and to a lesser extent the second, are currently being done at laboratories having large, powerful computers in their on-line data-acquisition systems.

Because computers have proved useful in so many fields, many varieties are now on the market, quite a few of them having properties highly suitable for nuclear-data acquisition. The properties particularly useful are, first, the ease with which a great variety of external input and output devices can be attached (interfaced to the computer); second, provisions for rapid, efficient response to interrupt signals from external devices; and third, usually a means of transferring data from external devices directly into blocks of memory without use of the central processor, the transfer possibly requiring only a single memory cycle per word. (This is referred to as direct memory access through a direct data channel.)

Several types of small computers have appeared on the market during the past year, some having 8-bit words, but they are too small for general data-acquisition use, although valuables for special applications. For present purposes, the smallest useful machines have a minimum memory size of 4096 (4k) 12-bit words, which can usually be enlarged to 32k words by the addition of memory modules, while the larger machines have minimum memories of at least 8k, with provision for expansion to several hundred k. Regardless of their size, the machines of the present generation all have memory cycle times around 1 or 2 µsec.

2. Rough Classification of Computers

Before proceeding with the discussion it is convenient to find a simple scheme for classifying computers. The scheme adopted here is to divide them into three loosely defined classes--small, medium, and large--essentially on the basis of the properties of the basic central processors:

Small Word length 12 to 18 bits Useful memory size 4k Number of bits in instruction 3 or 4 Floating-point hardware orally offered Approximate cost range $8500 to $40,000

Medium Word length 16 to 24 bits Useful memory size 8 to 16k Number of bits in instruction 4 to 6 Floating-point hardware option sometimes offered Approximate cost range $30,000 to $120,000

Large Word length 32 to 48 bits Useful memory size at least 16k Number of bits in instruction 7 or more Floating-point hardware Approximate cost range $150,000 or more

Computers do not fall neatly into these three classifications, especially since manufacturers offer many optional features; therefore, some argument about the assignment of a particular machine to one or the other class is possible. This is especially true with respect to the small and medium types. The properties of a large number of small and medium-sized computers are given in Appendix A. Information on larger machines can be found in the Adams Associates _Computer Characteristics Quarterly_.

D. MATCHING COMPUTERS TO TASKS

Having classified both the computers and the jobs that they may be called on to do, we now ask this question: How suitable is each of the three types of computers for each of the three classes of jobs, given that in every case the acquisition system consists of a single computer coupled to all necessary input and output equipment?

We start with the large computer system. All classes of jobs can be handled by this powerful system. However, we should question the wisdom of assembling a system based on a large machine unless a substantial amount of numerical calculating is anticipated, because the essential advantage of the large computer--the advantage that costs so much--is its capacity for rapidly executing highly accurate floating-point arithmetical operations.

The small computer system can handle the jobs of data acceptance, data manipulation, and output characteristic of the simple Class 1 operations, but they are suitable for very few jobs involving floating-point arithmetic. In fact, we must usually be skeptical about the use of small machines for any of the Class 2 operations except those of the process-control type, which in many cases would involve little if any arithmetic. (Process-control applications have been rather few to date, but a rapid increase can be expected in this field, especially because of the convenience and low cost of small modern computers.) It is apparent that these machines have been designed as economical instruments specifically intended to handle Class 1 jobs. The smallest word length of a machine in this group, 12 bits, is sufficient for storing in one word the output of a 4096-channel ADC unit, but it is not quite so convenient for handling the output of a typical scaler, which would likely require the use of two words. The capability of even a small computer system to convert experimental information into digital form, to transfer it into memory, to manipulate it, and to present it for inspection in a digested, convenient form, all at a high rate and essentially without error, is of immense value to an experimenter who has to cope with the abundant outflow of data from a modern nuclear experiment.

3. Medium-Sized Computers

The capabilities of medium-sized computers are less clear. These machines are superior to the small ones mainly in two respects: they have a more flexible command structure (i.e., they have a larger set of wired-in operations), and, usually, they have a longer word length. These features make them easier to program and give them a limited, but important, capability to execute floating-point operations sufficiently quickly and accurately for many purposes, even though these operations must in most cases be programmed, in the absence of floating-point hardware. We can reasonably conclude that the medium-sized machines will serve for any use listed in Classes 1 and 2. Certain simpler calculations of Class 3a are also expected to prove feasible, but few, if any, of those of Class 3b.

E. ON CHARACTERISTIC FEATURES OF COMPUTERS AND RELATED EQUIPMENT

The value of any feature depends on its need in the application involved; therefore detailed, absolute statements regarding each characteristic usually cannot be made. However, the Panel has discussed various features at some length, and we present here some general comments on the pros and cons of these features. Among the items discussed are some, such as word length and cycle time, that represent basic, inherent properties of the computer; while a great many others, such as priority interrupts, are customarily offered as options.

The shorter the word length the cheaper the hardware, generally speaking, but the less the accuracy in calculations unless multiple precision is used. For example, although the 12-bit words of the PDP-8 match the accuracy of data from most ADC's, they are too small not to match the output data from most counters; furthermore, indirect addressing is often required because a single word is too short to include both the operation code and the absolute address of a memory location. Apart from addressing considerations, a 12-bit word is too small for many uses, e.g., in general-purpose pulse-height analyzer applications where 16 bits or, better, 18 bits should be considered a minimum. Fortran programs for numerical calculations are in general best run on machines having at least 32-bit words, although 24-bit words are usually acceptable here when double precision can be used.

2. Number of Memory Words

In general the more words that a system can retain the better; but the greater the memory, the greater the expense. The cost must be weighed against the need. For simple handling of data, a 4k memory may be adequate, but in a large shared-time general-purpose machine a 16k or greater memory is essential. In the latter case, the resident shared-time monitor will probably occupy at least 6k of the memory, so with a 16k memory only 10k would be left accessible to users, and experience has shown that this much can be taken up completely by one user compiling a Fortran IV program. A 4k memory is adequate for many process-control applications, but it is too small for many other applications such as general-purpose pulse-height analyzer use, where an 8k memory is highly desirable. Adding a supplemental rotating memory device (disk or drum), at a cost per word about 1 percent that of core storage, is often preferable to adding core memory. See 6 below.

For most purposes the typical memory cycle time of 1 to 2 µsec is quite adequate. Some of the modern computers have cycle times under 1 µsec.

4. Direct Data Channels

These allow sequential depositing of digital data from external devices directly into blocks of computer memory without intervention of the central processor (direct memory access, DMA). Such input may require only one computer cycle per word, that being the next cycle after the one during which the interrupt signal arrives. This is the fastest means of getting data into memory, but it requires more external hardware and more complex interfacing than input through an accumulator of the central processor. Most data-acquisition machines provide both possibilities. Direct data channels can be valuable for interfacing to magnetic disks, drums, and tapes.

5. Priority Interrupts (Nested)

These can be very useful. They may cost as little as $125 each, depending on the machine, and can be used to reduce greatly the overhead running time losses of the computer. In complicated data-taking applications many interrupt lines are desirable; 8 to 16 priority levels are generally adequate. The usual Fortran compiler cannot compile programs that respond properly to interrupts, although a relocatable object code generated by the compiler can always be assembled with a machine-language subroutine designed to handle interrupts. Enlargement of Fortran compilers for data-acquisition use to include statements designed to handle interrupts is desirable. (See, for example, the discussion of the Yale-IBM system, Chapter 2, Section E.)