On-Line Data-Acquisition Systems in Nuclear Physics, 1969: A SMALL TIME-SHARED DATA-ACQUISITION

In 1966, two identical computer systems based on PDP-7 computers were set up in two different locations at the Lawrence Radiation Laboratory (LRL), to be used by several groups of experimenters (see Figure 2). Assembly of the hardware for the first system was completed 6 months after delivery of the computer. Assembly of the second system required only 3 months. Two years after operation commenced, the first satisfactory time-sharing monitor was completed and put into service. The basic use of these systems is pulse-height analysis. In principle many other types of operation are possible.

FIGURE 2 PDP-7 data-acquisition system at Lawrence Radiation Laboratory

FIGURE 3 A switch panel used for data taking and control of CRT display in conjunction with the PDP-7 computer. The switch-setting codes can be read into the PDP-7 accumulator under program control and are used to select branch points in the program. As many as eight of these units can be connected to the system. The lights are used to indicate program status.

Data-reduction jobs currently possible in the shared-time operating mode include spectrum stripping, normalization, smoothing, storage and retrieval of data from magnetic tapes, graph plotting, printout, energy calibration, background fitting, peak integration, and transfer of data from a remote analyzer. Remote control of the computer from up to eight experimental locations is possible using inexpensive switch panels (Figure 3). Remote slave CRT display is also provided. Multiparameter pulse-height acquisition and analysis can be done on a time-shared basis but often requires all the computer's time and memory.

The hardware configuration is shown in Figure 2. The PDP-7 computer was supplied by the Digital Equipment Corporation with an 8k memory (18 bits) extended arithmetic hardware, microtape (Dectape), paper tape, teletype, and cathode-ray tube (CRT). The other items were built or interfaced at LRL.

Automatic memory increment and memory-protection hardware, together with suitable programming, allow a user to carry out simple data-reduction jobs with a live CRT display while two other users are independently acquiring separate, 2048-channel, pulse-height spectra in part of the computer memory, with computer-controlled-gain stabilization. ADC dead time per pulse is less that 40 µsec. Up To 6144 words of the memory can be used for data (one PHA channel per word), while machine language programs fill the remaining 2048 words of memory.

The system works well for pulse-height analysis, but for new applications, e.g., nuclear magnetic resonance magnet control, it needs additional hardware and programs.

Two groups of experimenters, doing chiefly pulse-height analysis experiments are very satisfied with the system. Another group, with a wider range of interests, has been dissatisfied because of the time lag to implement new experiments.

One programmer is now engaged full time preparing more programs.

Lack of free computer time has become a limitation for both users and programmers.

Provision for programming at the outset was inadequate. One full-time systems programmer should have been assigned to these systems for 18 months.

Experimenters need Fortran or similar language capability. A disk, or more core memory, would make this practical.

The memory size is totally inadequate for multiple users because of the large amount of data space needed for the high-resolution spectra now obtainable with Ge(Li) detectors. An external 8k memory is being acquired for data acquisition in each system so that more of the computer memory can be used for computing.

Memory CRT's are needed to provide independent displays for each user. A separate teletype for each user would be invaluable.

A disk memory is needed for rapid overlay of programs and for sorting of multiparameter data.

A "czar" should have been appointed for day-to-day assignment of facilities, consultations with users, and routine maintenance and upkeep of the hardware and programs. The "czar" could be a good electronics technician interested both in programming and in physics.

System costs for the PDP-7 with time-sharing are given in Table 1. Fabrication time is included as a dollar cost. Engineering and programming times shown are one half those charged against two identical systems.

Additional special-purpose experimental equipment commonly used with the system includes gain stabilizers, analog pulse derandomizers, amplifiers, pulse pileup rejection, low-noise preamplifiers, and Ge(Li) detectors.