View source for Modified Modular Jack

{{Short description|Serial port connector type}}
{{Infobox connector
|name=Modified Modular Jack
|type=[[RS-423]] serial
|image=RJ DEC Type.jpg
|logo=
|caption=MMJ socket and plug
|designer=[[Company:Digital Equipment Corporation|Digital Equipment Corporation]]
|design_date=
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|production_date=
|superseded=
|superseded_by=
|superseded_by_date=
|external=
|hotplug=
|length=
|width=
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|electrical=
|ground=
|maximum_voltage=
|maximum_current=
|audio_signal=
|video_signal=
|data_signal=
|data_bit_width=
|data_bandwidth=
|data_devices=
|data_style=
|cable=
|physical_connector=
|num_pins=
|pinout_col1_name=
|pinout_col2_name=
|pinout_image=[[File:MMJ-socket pinout.svg|150px]]
|pinout_caption=
|pin1=DTR
|pin1_name=Data terminal ready
|pin2=Tx+
|pin2_name=Transmit data
|pin3=Tx-
|pin3_name=Transmit data ground
|pin4=Rx-
|pin4_name=Receive data ground
|pin5=Rx+
|pin5_name=Receive data
|pin6=DSR
|pin6_name=Data set ready
|pinout_notes=
}}
The '''Modified Modular Jack''' ('''MMJ''') is a small form-factor [[Serial port|serial port]] connector developed by [[Company:Digital Equipment Corporation|Digital Equipment Corporation]] (DEC). It uses a modified version of the 6P6C [[Modular connector|modular connector]] with the latch displaced off-center so standard modular connectors found on [[Ethernet]] cables or phone jacks cannot accidentally be plugged in. MMJ connections are used on Digital [[Minicomputer|minicomputer]]s, such as the [[Engineering:PDP-11|PDP-11]], [[VAX]] and [[DEC Alpha|Alpha]] systems, and to connect terminals, printers, and serial console servers.<ref>Andrew Oliviero, Bill Woodward ''Cabling: The Complete Guide to Copper and Fiber-Optic Networking'',
John Wiley & Sons, 2009, {{isbn|978-1-118-80734-7}}, page 295 </ref>

The MMJ connector has six conductors, using a superset of the [[RS-422]] serial communication standard. The six pins are Tx and Rx for the data transmission, their return paths, 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 signaling by combining the lower voltage sides of each signal to the RS-232 signal ground line.

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

Thrustmaster is 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.{{cn|date=August 2019}}

The [[Lego Mindstorms]] sets, beginning with the NXT, used similar plugs for motors and sensors to prevent the connection of standard telephone cords.{{cn|date=August 2019}}

CEA-909 uses the MMJ connector for control signals to a smart antenna, which can either physically or electrically rotate for maximum signal strength.

==See also==
* Serial console

==References==
<references/>
== External links ==
*[http://www.lammertbies.nl/comm/cable/dec-mmj.html Wiring schemes for Modified Modular Jack cables and converters]

[[Category:Networking hardware]]

{{Sourceattribution|Modified Modular Jack}}