M753

This document is a proprietary schematic diagram for the Digital Equipment Corporation (DEC) M753 Asynchronous Modem Line Control module, copyrighted in 1968 and intended for test and maintenance purposes.

The circuit is designed to manage and control various signals for two asynchronous modem channels (A and B). Key signals and functions include:

• Request To Send (ROST TO SEND A/B)
• Terminal Ready (TERM RDY A/B)
• Carrier Change (CAR CHA/CHB)
• Ring Flag (RING FLAG S1, RING A/B)
• Carrier (CAR A/B)
• Status and Control Flags (C FLAG, C STATUS, INITIALIZE)

The design primarily utilizes a variety of DEC 7400-series integrated circuit logic gates:

• DEC7474N (D-type flip-flops) for E1, E5, E9.
• DEC7402N (NOR gates) for E4, E7.
• DEC7401N (NAND gates, likely open-collector) for E8, E11.
• DEC7400N (NAND gates) for E2, E3, E6, E10.

It also incorporates discrete components such as resistors (1/4W, 5%), capacitors (100V, 5%), and D662/IN645 diodes. The module operates with +5V, +3V, and +2JV power supplies, with Pin 7 of each IC connected to ground (GND) and Pin 14 to +5V.