Circuit Board Construction
Each circuit board was four inches wide and eight inches long. Each board had six layers. The two outside layers, referred to as the vertical and horizontal layers due to the orientation of their foil runs, carried logic signals between the integrated circuits on the board. Logic signals between the back-panel connector and the gate array chips also used these two foil layers. The inner board layers were used to distribute power, system clock, and ground to each chip location. There were two power layers, -5.2 volts and -2.0 volts, one system clock layer, and one ground layer. The system clock layer was used, on a very limited basis.
Circuit Board Power and Ground Connections
At one end of each printed circuit board the power and ground foil layers were connected to foil pad areas on the top and bottom surfaces of the board. The power and ground bus lead connection blocks were inserted between the boards at the pad areas when the eight boards are assembled to make a complete module. The power bus blocks served a dual purpose in the module as spacers between boards and as power bus connections. Each module had seven power bus leads to be connected to the -5.2v system bus bar, seven for the -2.0 power, and fourteen ground leads.
Circuit Board Input/Output Signal Connections
At the end of the circuit board opposite the power connections, a 72 pin connector was mounted serving as both an input/output signal connector and as a board spacer when the module was assembled. This connector was divided into three sections of 24 pins each, with the three sections being assigned the same letter designations as the chip fields of the board on which they were mounted, as described in the "The CRAY-2 Module" post. The 24 pins in each section were labeled (A through X). This allowed reference to any one of the 576 pins on the module using a simple two letter designator, the first being the connector section and the second indicating the pin within the section.
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