On-Line Data-Acquisition Systems in Nuclear Physics, 1969

Chapter 1. The Tasks and the Computers

THE TASKS AND THE COMPUTERS

A. INTRODUCTION

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.)

B. THE TASKS

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.

C. THE COMPUTERS

1. Introduction

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?

1. Large Computers

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.

2. Small Computers

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.

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