5287534 "Correcting Crossover Distortion Produced When Analog Signal Thresholds Are Used To Remove Noise From Signal" It describes the DEC CXM04 board for the DS550 communications server, which inserts itself between an IBM establishment controller and a control unit (coax) terminal so the CUT can pretend it's a serial terminal to VAXen without dropping the polled connection to the IBM mainframe. weird..