Modified Modular Jack
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The DECconnect Modified modular jack (MMJ) is a variation on the more standard 6P6C modular connector made by Digital Equipment Corporation to connect to their mainframe computers. The main difference from the conventional modular connector is that the hook is toward the side instead of the center of the plug. The Shape of the MMJ makes it impossible to plug a standard modular cable, such as for telephone or Ethernet, into an MMJ. The MMJ's were also used on DEC minicomputers (such as PDP-11, VAX and Alpha based machines) to connect DEC terminals or serial console servers.
The six conductors of the DECconnect MMJ connector are used for the main signals in RS-232 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 DEC sold a number of models of conversion adapter.
For connection of two DTE devices (computers, printers, etc) the BC16E connection cable is available which automatically crosses the pins. Therefore the DEC MMJ system has a well defined way for null modem communication, something which is somewhat problematic with the conventional RS-232 deployments.
