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

by National Research Council . Ad Hoc Panel on On-line Computers in Nuclear Research

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.

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.

1. Word Length

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.