On-Line Data-Acquisition Systems in Nuclear Physics, 1969: Chapter 2 - MULTIPLE-COMPUTER SYSTEMS

On-Line Data-Acquisition Systems in Nuclear Physics, 1969, by H. W. Fulbright et al. National Research Council is part of the HackerNoon Books Series. You can jump to any chapter in this book here. Chapter 2: MULTIPLE-COMPUTER SYSTEMS

At the Rutgers-Bell (RB) nuclear physics laboratory, work has been done with two different two-CPU systems. The first of these represented essentially two duplicate processors (Figure 8), and the second, now in the process of implementation, two processors of different size and capability (Figure 9). While full data are not yet available on the actual performance of the second system, an outline of the experience to date will be given.

FIGURE 8 The two-central-processor system of Rutgers-Bell.

The initial success of the original RB SDS 910 data-acquisition system was soon tempered by a result of its popularity: during most experiments the computer was unavailable for program development or data analysis. Since most experiments required the use of displays and light pens in at least one stage of data analysis, the computer center could not handle the work.

FIGURE 9 The new Rutgers-Bell Sigma 2-Sigma 5 system.

The solution adopted was to acquire another computer with the same instruction set (an SDS 925) and to provide switches such that the line printer, card reader, and plotter could be run from either computer. No provision was made for direct transfer of data from one computer to the other.

In practice this system worked out quite well. There was complete interchangeability of programs from the 910 to the 925, which differed only in being five times faster. Normally the switchable peripherals were run from the 925; when the group taking data wished to print or plot current spectra, they consulted with the 925 users, then used the peripherals with little more difficulty than permanently attached units would have involved.

A further advantage of the switchable peripherals, in addition to the cost saving, was that the experiments associated with the 910 could proceed while the peripherals were being serviced. The 910 is exceedingly reliable, averaging less that one main frame failure per year, and the 925 is nearly as reliable. The vast majority of service calls have been occasioned by the peripherals and have competed with data analysis but not with accelerator utilization.

In addition to the switched peripherals, both computers were equipped with two magnetic tape transports, electric typewriter, and high-speed paper-tape reader and punch. While these units were also subject to downtime, the paper-tape system and the typewriter could be exchanged between the 910 and 925. Only the magnetic-tape transports required the use of the 910 CPU during servicing, and the presence of two transports has usually meant that the second one could carry the load until the weekly accelerator maintenance period.

While the reliability record of the central processors has been excellent, that of many of the peripherals has not. Here is an excellent justification for renting computing equipment: if units do not work well, they can be returned. For a time, a low-cost card reader (100 cards per minute) built by NCR for SDS was used. It was unacceptable in reliability and was replaced by the Univac reader which came with the 925. Another unit returned was a cartridge magnetic-tape system built by SDS. The Ampex TM-4 magnetic-tape transports on both the 910 and 925 have been consistently poor in reliability, but no other unit has been available to replace them. A manufacturer's name does not seem to be a guarantee of good or bad quality—the line printer, also made by NCR, has been excellent both in reliability and print quality.

While the two-computer system generally rated high in user satisfaction, considerations of performance have led to the design of a larger and more powerful system with totally new components. The 925, without wired multiplication or floating-point operations, was too slow for theoretical computation or for many types of data analysis such as those using Monte Carlo methods. Interactive methods of analysis, using a display and light pen, have been found very effective in the cases where the 925 could accommodate them but have not been available through either the Bell Laboratories or Rutgers computer centers.

A further limitation on the earlier system was that only one person could use the 925 at a time. The generation of a display involved the full time of the CPU, and while multiprogramming might have been able to divert some CPU time, the 8k memory size did not permit it.

Data acquisition on the 910 was limited in array size to the capacity of the core memory. For multiparameter experiments, three, six, or even twelve 4096-channel arrays have been stored in core, but the advantages of live display available with core storage have discouraged anyone from handling large arrays by logging raw data on magnetic tape for analysis later. Memory expansion would have been desirable, but the necessity of making the expansion on both the 910 and the 925 effectively doubled the cost.

Limited flexibility, then, is a major drawback of this type of system. As long as only two users needed to be accommodated, and each could adapt to exactly half of the total core storage, it was satisfactory and provided redundant facilities to guard against experiment downtime due to computer failures.

In ordering a new computer powerful enough to handle most of the nuclear physics laboratory's data analysis and theoretical computing tasks, cost ruled out the acquisition of a pair of program-compatible computers. It was recognized that desirable features of the original system would have to be obtained in new ways. Accessibility of the system for programming could be improved by running a simple time-sharing monitor on it. Reliability could be enhanced by avoiding bargain peripherals and using only items of demonstrated high quality and by the capability of running the peripherals on either computer.

The use of a separate CPU for data collection still seemed particularly desirable, however. A combination of a large (by present standards) computer with a powerful small computer as a front end was designed. It includes a display disk for refreshing displays without CPU attention, as well as for storing data arrays too large to be kept in core. The computers selected were a 32k, 32-bit SDS Sigma 5 and a 12k, 16-bit Sigma 2.

The new system, with separate and nonequivalent computers, will have advantages over the old system in data analysis and general computation, because these will be done on the larger computer, either in time sharing or batch mode. Time sharing should enhance the flexibility of the system by making it easier to generate and debug new programs, in addition to improving the accessibility.

For the data-collection computer, RB will lose the advantage of a separate computer on which complete debugging of programs may be done. This loss can be tolerated since the fraction of the load carried by the Sigma 2 will be less than that carried by the 910 in the old system. In the old system, very few distinct data input or display programs were written. A few subroutines and their calling parameters sufficed for all needs for six years; the logic and I/O operations unique to each experiment were written in Fortran by the experimenters.

In the new system, the Sigma 2 will be concerned with the operations used in the data acquisition and formating of displays; most of the rest can be left in the Sigma 5, with routines sent over to the Sigma 2 under the time-sharing system. If the user should prefer, he can operate the Sigma 2 directly and make use of the Sigma 5 only for data storage.

Until very recently, program development on the Sigma 2 has been slow because it lacked means of getting program listings quickly. We have now developed an assembler for the Sigma 2 which runs on the Sigma 5. The availability of card reader input and line printer output has greatly speeded Sigma 2 software development. The loading of Sigma 2 programs is also much more convenient, since they can be stored on the Sigma 5 disk and loaded exactly as Sigma 5 programs. It seems highly desirable to have an assembler for any small data-acquisition computer capable of running on another machine; the means of transporting the object code to the small computer is of less importance.

The reliability of the new equipment has been excellent. Only the card reader has had any downtime of consequence, and modifications seem to have resolved its problems. The Sigma 5 main frame has had no failures in 12 months, and the Sigma 2 has had only one in the past year. If this record continues, the loss of the redundancy inherent in the old 910/925 system will not have any serious effects.

One component of the new system is taking on an increasingly important role, although it had not been a part of the original planning. That is the computer-independent data bus consisting of system controller, bin controller, and register units. Only the system controller is specific to a particular computer; moreover the same system controller design could be used on both the Sigma 2 and Sigma 5 by restricting the data path to 16 bits.

The register units are used to interface external devices to the computer quite cheaply; a typical register used here to interface an existing Calcomp plotter to the new computers costs about $300 in parts and labor. Similar units are used to interface the Sigma 5 to the Sigma 2 and to the 910, to drive a temporary core-resident display on the Sigma 5, to read pushbutton inputs, and to read ADC's. The display disk controller now under construction uses these registers to furnish control information, although the data go directly to and from the core.

At the present time, the registers are read and written under program interrupt control, but the design is not limited to program-controlled operation. By substituting a controller designed to operate automatically (directly to core or to the I/O processor) speeds approaching 1 or 2 µsec per word transferred could be obtained. Such interfaces have been built for various computers using the European CAMAC bus system, which is conceptually similar.

The system is highly modular and is built into NIM bins with modified back connectors. Exchange of modular units has been very helpful in debugging the system, and presumably it will also be helpful in case of failures in operation. This is a much more satisfactory situation than that which was obtained with the ADC interface on which RB collaborated with Brookhaven. The latter unit was built with computer-type construction: commercial logic cards and wire-wrapped back panel. Debugging of that unit was exceedingly laborious because of the lack of modularity in its components.

The computer-independent bus system has not been expensive in manpower. It has required about 9 man-months in design and debugging and somewhat less time in construction. The registers cost about $300, as mentioned, and the controllers $1500 to $2000 depending on the need for cable drivers.

The costs of the RB multiple-computer system are given in Table 6. The figures are approximate and not the result of detailed accounting.