when a button is pushed. The storage-tube scheme is probably the best buy for use in a typical small system. The Tektronix Company has recently announced a storage-tube device, Type 4501, which is said to generate a continuous video signal suitable for driving large-screen television monitors.

A fourth scheme involves the generation of a video (analog) signal corresponding to the display, written on a disk or drum by the computer. Reading heads then send the video information to a CRT having a TV raster synchronized with the rotation of the medium. This is a good scheme where many displays are needed, but it is too expensive for many applications, costing upwards of $20,000 for the first unit. (For example, the Data Disc System 6500 Display costs about $23,000.)

One display feature considered desirable by many nuclear physicists is rotation of isometric data plots. This can be accomplished in one of two ways: recomputing every displayed dot or using an appropriate analog device (potentiometer). Because the latter is so cheap, clearly its use is more desirable than the recomputation of the rotated view. Also, using a light pen on a recomputed display is especially difficult because the inverse computation has to be performed in order to maintain proper correlation with the original data. However, it should be noted that the TV raster technique is limited in this respect: rotating potentiometers cannot be used, and the image must be recomputed. The technology of displays is developing rapidly.

16. The Role of External Devices

In many cases, especially where typical standard operations are involved, it is preferable to use external devices to handle preliminary selection and sorting of events, rather than to ask the computer to do the entire job. For example, particle identification by use of signals from two counters involves one or two multiplications and additions, which can be carried out almost instantly by a fairly simple external analog device, whereas a small computer would likely require at least 500 µsec for the job, assuming calculation, and perhaps 40 µsec, assuming table look-up.

17. Time Sharing

Computers as small as a PDP-8 have been successfully time-shared by several users in special applications. The justification given is that all the peripheral hardware can be shared also, so that the added constraints and programming difficulties are balanced by savings in hardware costs. Computers have also been shared for simultaneous on-line data-taking in low-data-rate experiments. In working out the economics of time-sharing, the added hardware (such as CRT's and remote consoles and memory protection) needed to allow simultaneous access by more than one user, as well as the extra memory space needed by the time-sharing monitor, should be considered. The greatest costs, however, lie in the added constraints placed on each of the users and in the greatly increased cost of programming. In many cases the use of two or more identical computers is preferable. However, in large, expensive systems time-sharing can be very useful.

18. Software That Should Be Supplied by Manufacturer

Complete documentation should be provided, including listings, step-by-step user instructions, and some fully worked out examples.

a. Hardware diagnostic routines: To test memory addressing, instruction set and to test correct operation of every peripheral and special hardware feature.

b. Systems to edit, assemble, and debug programs in symbolic machine language: These should efficiently use any special I/O device such as magnetic tape, disk, or line printer.

c. Efficient subroutines should be provided for operation of any special peripheral device purchased from the computer manufacturer. Symbolic language source tapes or card decks, listings with comments, and examples of use should be included.

d. Conversational Fortran-type programs provided by some manufactures are useful for supplemental calculations.

NOTE: The following points apply particularly to the medium and large machines and become increasingly important as the computer becomes larger and more complex.

e. Fortran compiler and operating system, with convenient method to insert machine language instructions and subroutines. Good compile and run-time diagnostics are essential.

f. Mathematical subroutines should be provided in binary and source language.

g. Complete specifications and documentation for the programming system should be supplied, so that programs prepared by users can be made compatible. It may be objected that this will cost too much, but not to do so will be very costly and frustrating to many users.

19. Note on the Cost of Programming

Experience at Brookhaven and Berkeley has shown that a programmer can produce between 10 and 20 debugged and documented lines of program per day, depending on such factors as experience, when he is working on reasonably straightforward programming. When working on a complicated monitor system he would be considerably less productive. System programming is obviously very expensive, therefore the average person exploring the computer market would be well advised to consider the software support along with the hardware offered in each case. Manufacturers vary greatly in this respect. A major contributing factor to the persistent popularity of the PDP-8 is that the software support is so extensive. In general, the newer a computer, the less software is likely to be available.

Chapter 2
DATA-ACQUISITION SYSTEMS

A. INTRODUCTION

1. History

The movement toward computer systems began in earnest about 1962. Much of the early work depended on the use of magnetic tape for storage of data, either raw or partially digested, the analysis of data being carried out later, off-line. More recently, computers have been used increasingly for on-line processing. The early work is well known and will not be described here. Some of the more recent systems are basically very close descendants of one or another of the early systems. Many varieties are now in service. Most incorporate small or medium-sized computers, however, extensive new experience has been gained during the past two or three years of operation of a few large time-shared systems, in particular those in the tandem Van de Graaff accelerator laboratories at Yale and at Rochester, perhaps the first large systems in operation which were planned systematically for nuclear research. Both operate with multiprogramming monitor control, background calculations being possible, on a low-priority basis, simultaneously with data acquisition.

2. Possible Systems

Simple rules for the design of various types of data-acquisition systems cannot be stated, but some examples of possible systems can be given. (See Figure 1.)

a. A simple system for pulse-height analysis work can be assembled from a small computer, a 5-in. Tektronix CRO, an ADC unit, and a teletype with paper-tape attachment for a cost of about $30,000, providing that a competent engineer is available, not counting programming and engineering costs. A Calcomp plotter could be added for about $6000. To maintain and operate the system at least a half-time technician-programmer would be required.