The 3B20D Processor & DMERT Operating System: 3B20D Input/Output System

The 3B20D Input/Output system was planned from conception to be very flexible, and to support a large number of different peripheral devices. To accomplish this, several difficult system and design-level problems had to be solved. (i) Processor real time--Previous experience in system input/output had shown that even low-speed input/output could be very realtime intensive. Terminal input/output, in particular, requires considerable per-character processing. The 3B20D Input/Output system, therefore, had to provide for front-end processing. (ii) Lack of interface standards--Input/output standards at the physical, electrical, and protocol levels for certain peripheral devices, even those of the same functional class, i.e., tapes, floppy disks, and so 255 on, are relatively nonexistent. Where there are standards, such as Electronics Industries Association's RS 232 (electrical and physical standards for data link and terminal access), they often are interpreted differently by various vendors, cover only a portion of the market for a class of devices, or are subject to periodic revision. The input/output system, therefore, had to provide a standard interface that could easily support a wide range of nonstandard interfaces and power/ground systems. (iii) Variations in operation and maintenance processing--Error handling, as well as operational processing, is significantly different for each peripheral device. Noninterruptive growth and maintenance--The input/output system had to permit easy addition of new or replacement devices without interrupting service.