TESTER FOR COLLISION-DETECT CIRCUITRY

This document is a United States patent (No. 4,380,088) assigned to Digital Equipment Corporation, describing a collision-detect tester for transceivers used in multi-access data communications networks (specifically CSMA/CD networks).

The patent details a mechanism that automatically tests the collision detection circuitry of a transceiver at the end of every transmission. By monitoring the transmitter's squelch signal, an "end-of-transmission" detector triggers a "collision simulator" that injects a signal into the receiver, mimicking a collision. If the receiver is functioning correctly, it will report the collision to the host system. Since the host is aware that the signal occurs immediately after a transmission, it can distinguish this test signal from a real network collision. If no signal is reported, the host identifies a fault in the collision detection circuitry, allowing the transceiver to be disabled before it causes network issues. The patent also covers related features, such as isolation circuitry to prevent short circuits and guard circuitry to ensure a transmitter does not get "stuck on" and monopolize the network.