Modified Modular Jack

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Modified Modular Jack
MMJ-socket pinout.svg
RJ DEC Type.jpg
MMJ socket and plug
Type RS-423 serial
Designer Digital Equipment Corporation
Pin 1 Data terminal ready DTR
Pin 2 Transmit data Tx+
Pin 3 Transmit data ground Tx-
Pin 4 Receive data ground Rx-
Pin 5 Receive data Rx+
Pin 6 Data set ready DSR

The Modified Modular Jack, also known as the MMJ connector, or more rarely DEC MMJ, is a small form-factor serial port connector developed by DEC. It uses a modified version of the 6P6C modular connector with the hook displaced to the side so standard modular connectors found on Ethernet cables or phone jacks could not accidentally be plugged in. MMJ connectors are used on Digital minicomputers, such as the PDP-11, VAX and Alpha based machines and to connect terminals, printers or serial console servers.

The MMJ plug has six conductors, using a superset of the RS-423 serial communication standard. The six pins were Tx and Rx for the data transmission, their ground wires, and DSR and DTR for handshaking. The transmit and receive signals are differential, i.e. each signal is the voltage difference between the line and its associated ground, as opposed to a voltage on a single connector relative to a common reference. The system can also emulate RS-232 signal by combining the lower voltage sides of each signal to the RS-232 signal ground line. For this type of connection there are a number of models of conversion adapter.

When connecting two DTE devices such as a computer and a printer, the Digital BC16E crossover cable is used.

Thrustmaster is also using the MMJ connector for connecting its line of rudder pedals and racing pedals to either USB via a proprietary Thrustmaster Adapter, or directly to a joystick or racing wheel.

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