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.
6. Computer-Independent Data Bus System
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.