On-Line Data-Acquisition Systems in Nuclear Physics, 1969: Chapter 2 - A SMALL SYSTEM BASED ON A PDP

This second example of a small computer system is also taken from experience at LRL. It was planned in February 1967 and first put into operation in the summer of 1967. Data were first taken with the aid of the system in the spring of 1968, and the system programming was completed in May 1969. The system is used extensively in experiments with the Bevatron.

FIGURE 4  PDP-8 data-acquisition system at Lawrence Radiation Laboratory.

The computer-system hardware consists of the items shown in Figure 4. The PDP-8 has 4k of memory. The disk is a Data Disc unit with a removable disk (128 tracks and a movable head) on the same shaft as a smaller disk and three fixed data heads. Two of the fixed-head tracks are devoted to the display: they drive a hardware-translator continuously. A single display track is used when the number of points does not exceed 2000. For larger displays two tracks are used alternately.

The display is controlled from the switch panel. On-line operating functions may be controlled both from the switch panel and from the teletype.

Three classes of programming have been completed:

System Programs: Symbolic Text Editor, Assembler, a general-purpose library system—all disk oriented.

Data-Taking Programs: These cause the ADC to be read, control elementary sorting, update histograms resident on the disk, write raw data on tape, and monitor the beam. The Bevatron has approximately 1 sec of beam every 6 sec. During a beam pulse the computer is devoted entirely to acquiring data, saving raw data in core, on the disk, and on tape. After a beam pulse, the 13-disk histograms are updated, then the display programs are read into core memory and the display is updated. The system is designed to be capable of accepting over 1000 events per beam burst, and it has met this requirement.

Simple Data-Analysis Programs: These compute displays (linear, log, isometric, and contour—all double precision), read out the sealers, monitor the real-time clock, allow resorting of raw data from tape, and generate tapes for remote plotting.

The system now functions as originally intended and does its job very satisfactorily.

The experimenter relies heavily on the main computer center for data processing.

In assembling this system now, one would buy the disk already interfaced by the computer manufacturer; furthermore the manufacturer now offers programs that would greatly reduce the programming costs.

A memory scope would eliminate the need for a disk-to-CRT display interface.

Less-expensive magnetic tapes are now available. However, it would be better to buy the tape already interfaced by the manufacturer of the computer.

The added cost of buying a 16-or 18-bit computer would have been almost completely offset by savings in the cost of programming.

The addition of a fast printer (e.g., Inktronic $5600) would have paid for itself in time saved during programming but would not be of much use in experiment.

The costs of the PDP-8 are given in Table 2.