Modified Modular Jack
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The Modified modular jack was made by Digital Equipment Corporation to connect their mainframes. The main difference with the Modified jacks was that the key which is not in the middle but in the side. The Shape of the MMJ makes it impossible to plug a RJ11 or a RJ45 cable into an MMJ . The MMJ's were also used on DEC minicomputers (such as PDP-11, VAX and Alpha based machines) to connect VT100 terminals or serial console servers.
The six available leads in the DECconnect MMJ connector are used for the main signals in serial communication: Tx and Rx for the data transmission and DSR and DTR for handshaking. The transmit and receive do not have a common ground, but they are differential, i.e. each line has it's relative ground level on a separate lead. By combining both floating minus leads and connecting them to the RS-232 signal ground at the other side, signals can in practice be exchanged with a normal RS-232 device. For this type of connection DEC sold a number of conversion adapters.
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 original RS-232 standard.
related links
External links
- [Wiring schemes for Modified Modular Jack cables and converters]
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