Modified Modular Jack
Encyclopedia
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 minicomputer
s, such as the PDP-11
, VAX
and Alpha
based machines and to connect terminals, printers or serial console server
s.
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.
Modular connector
Modular connector is the name given to a family of electrical connectors originally used in telephone wiring and now used for many other purposes. Many applications that originally used a bulkier, more expensive connector have now migrated to modular connectors...
and was developed by Digital Equipment Corporation
Digital Equipment Corporation
Digital Equipment Corporation was a major American company in the computer industry and a leading vendor of computer systems, software and peripherals from the 1960s to the 1990s...
. 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 minicomputer
Minicomputer
A minicomputer is a class of multi-user computers that lies in the middle range of the computing spectrum, in between the largest multi-user systems and the smallest single-user systems...
s, such as the PDP-11
PDP-11
The PDP-11 was a series of 16-bit minicomputers sold by Digital Equipment Corporation from 1970 into the 1990s, one of a succession of products in the PDP series. The PDP-11 replaced the PDP-8 in many real-time applications, although both product lines lived in parallel for more than 10 years...
, VAX
VAX
VAX was an instruction set architecture developed by Digital Equipment Corporation in the mid-1970s. A 32-bit complex instruction set computer ISA, it was designed to extend or replace DEC's various Programmed Data Processor ISAs...
and Alpha
DEC Alpha
Alpha, originally known as Alpha AXP, is a 64-bit reduced instruction set computer instruction set architecture developed by Digital Equipment Corporation , designed to replace the 32-bit VAX complex instruction set computer ISA and its implementations. Alpha was implemented in microprocessors...
based machines and to connect terminals, printers or serial console server
Console server
A console server is a device or service that provides access to the system console of a computing device via networking technologies....
s.
The six conductors of the DECconnect MMJ connector are used for the main signals in RS-423
RS-423
RS/EIA/TIA-423 is a standard for serial communications. It defines an unbalanced interface , with a single, unidirectional sending driver, and allows for up to 10 receivers . It is normally implemented in integrated circuit technology and can also be employed for the interchange of serial binary...
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
Crossover cable
A crossover cable connects two devices of the same type, for example DTE-DTE or DCE-DCE, usually connected asymmetrically , by a modified cable called a crosslink...
is used.