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 DECconnect Modified Modular Jack (MMJ) is a variation of the 6P6C modular connector and was developed by Digital Equipment Corporation. The main difference from the conventional modular connector is that the hook is toward the side instead of the center of the plug, making it impossible to insert a standard modular cable plug such as those used telephone or Ethernet applications. 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 six conductors of the DECconnect MMJ connector are used for the main signals in RS-423 serial communication: Tx and Rx for the data transmission and DSR and DTR for handshaking. The transmit and receive signals are differential, i.e. each signal is the voltage difference between two conductors, as opposed to a voltage on a single connector relative to a common reference. But one can in practice wire these signals to an RS-232 device, which uses the common reference method, 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.

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