472 Part 2 ½ Regions of Computer Space Section 6½ Fault-Tolerant Systems
supply. If a failed module is to be replaced in this section its associated power supply is shut off, the module is replaced, and the power supply is turned on. Each card cage slot in the 110 card cage is powered by two different power supplies. Each of the I/O controllers is connected via its dual-port arrangement to two processors. Each of those processors has its own power supply; usually, but not necessarily, those two supplies are the ones that power the I/O controller (Fig. 3). Each slot in the I/O card cage can he powered down by a corresponding switch disconnecting power from the slot from both supplies without affecting power to the remainder of the system. Therefore, if a power supply fails, or if one is shut down to repair a processor, no I/O controllers are affected.
The dual-power sourcing to the I/O controllers was originally designed using relay switching. This plan was abandoned for several reasons: (a) to contend with relay failure modes is difficult; (b) the number of contact bounces on a switch-over is neither uniform nor predictable, making it difficult for the operating system to handle power-on interrupts from the I/O controllers; and (c) during the switch-over, controllers do lose power, and while most controllers are software-restartable, communications controllers hang up their communications lines. We therefore devised a diode current sharing scheme whereby I/O controllers are constantly drawing current from two supplies simultaneously. If a power supply fails, all the current for a given controller is supplied by the second power supply. There is also circuitry to provide for a controlled ramping of current draw on turn-on and turn-off so there are no instantaneous power demands from a given supply causing a potential momentary dip in supply voltage.
Both fans and power supplies are electrically connected using quick
disconnect connectors to speed replacement upon failure. No tools are required
to replace a power supply. A screwdriver is all that is needed to replace
a fan. Both replacements take less than 5 minutes.
Interconnections
Physical interconnection is done both using front edge connectors and back-planes. Communication within a processor module (e.g., between the CPU and main memory) takes place over four 50 pin front edge connectors using flat ribbon cable. Interprocessor communication takes place over the Dynabus on the back-plane also utilizing ribbon cable. The I/O controllers use etch trace on the back-plane for communication among PC cards of a multicard controller. The I/O channels are back-plane ribbon cable connections between the processors and the I/O controllers.
Peripheral I/O devices are connected via shielded round cable either to a bulk-head patch panel or directly to the front edge connectors of the I/O controllers. If a patch panel is used, then there is a connection using round cables between the patch panel and the front edge connectors of the I/O controllers.
Power is distributed using a DC power distribution scheme. Physically,
AC is brought in through a filtering and phase splitting distribution box.
Pigtails connect the AC distribution box to one of the input connectors
of a power supply. The DC power from the supply is routed through a cable
harness to a laminated bus bar arrangement which distributes power on the
back-planes to both processors and I/O controllers.
2. Processor Module Organization
The processor (Fig. 5) includes a 16 bit CPU, main memory, the Dynabus interface control and an I/O channel. Physically the CPU, I/O channel and Dynabus control consists of two PC boards 16 inches by 18 inches, each containing approximately 300 IC packages. Schottky TTL circuitry is used. Up to 512K bytes of main memory is available utilizing core or semiconductor technology. Core memory boards hold 32K 17-bit words, and each occupies two card slots because of the height of the core stack. Semiconductor memory is currently implemented utilizing 16 pin, 4K dynamic RAMs. These memory boards contain 48K 22-bit words per board and occupy only one card slot and are therefore three times denser than core.
The processor module is viewed by the user as a 16-bit, stack-oriented processor, with a demand paging, virtual memory system capable of supporting multiprogramming.
The CPU
The CPU is a microprogrammed processor consisting of a bank of 8 registers which can be used as general purpose registers, as a LIFO register stack, or for indexing; an ALU; a shifter; two memory stack management registers; program control registers