Digital Equipment Corporation

Roger A Dame

Darrel D Donaldson

Ronald E Nikel

An open drain driver, differential receiver circuit for transmitting logic signals generated by metal oxide semiconductor (MOS) logic circuits over a data bus that uses distributed termination impedances. The distributed termination impedance values are chosen to minimize the data bus settling time both when a driver in a given module drives a signal consisting of alternating logic levels, as well as when two different drivers drive a given logic level in succession. Each open drain driver includes a multiplexor, a flip flop, an inverter stage, and a driver transistor. The multiplexor selects either a data signal to be transmitted or a deasserted data value to be passed to the input of the flip flop. The flip flop also accepts a test input, which sets the flip flop to a known state. As a result, no additional delay is inserted in the critical path between the rising edge of a flip flop clock input signal and the driver output. The companion differential receiver receives a reference voltage which is separately generated on each module, to permit the use of distributed power supplies while still adjusting for variations which may exist in a termination voltage from module to module.

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West Brookfield, MA, United States of America

Roger A Dame