MMJ Info

From: Mzthompson_at_aol.com <(Mzthompson_at_aol.com)>
Date: Thu Oct 19 10:54:38 2000

Info on the MMJ connector found on various DEC terminals & computers.
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DEC uses an MMJ (Modified Modular Jack) connector on its equipment for serial
data communications. DEC calls the mating plug on the cable an MMP
(Modified Modular Plug), only the term is not used as often and most of the
time is just simply referred to as MMJ. It is like a modular telephone
connector, only the key on the connector is offset, not in the center like
a standard telco connector.

Looking at the back of a VT320 for example the MMJ looks something like this.

   --------------------
   | * * * * * * |
   -------------- --
                |____|

     1 2 3 4 5 6 are the pinout numbers

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The signals are:

# I/0 Desc
1 > DTR Data Terminal Ready
2 > TXD Transmit Data
3 - TXD- (ie Gnd)
4 - RXD- (ie Gnd)
5 < RXD Receive Data
6 < DSR Data Set Ready

> = terminal or computer output signal
< = terminal or computer input signal

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DEC's part number for the cable is BC16E-xx, with the xx being
the length. The cable is made so that the connector on one end is
mounted 'right side up' and the other end 'upside down'. In other
words, the key tabs on the connector are on opposite sides of the
flat cable.

           //
           ---- ----
           | |---------------------------------------| |
           ---- ----
                                                        //

That is effect causes the signal lines to cross-connect or as I like
to say 'turn over' from one end to the other.

            MMJ port on DEC MMJ port on DEC
            VT320 terminal computer or DECserver

         DTR 1 --->-------------->----------------->--- 6 DSR
         TXD 2 --->-------------->----------------->--- 5 RXD
             3 ---------------------------------------- 4
             4 ---------------------------------------- 3
         RXD 5 ---<--------------<-----------------<--- 2 TXD
         DSR 6 ---<--------------<-----------------<--- 1 DTR

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I had a DEC MMP cable that someone cut one end off so I wired it to a female
9-pin D connector to use with a PC 9-pin serial port. Wired as follows:

MMJ DE9
 1 6
 2 2
 3 5
 4 5
 5 3
 6 4

This was to use a PC as the console on a DECsystem 5500 and/or one of the
ports on a DECstation 3100. I actually did not connect DE9-6 to MMJ-1.
It worked fine without it. I later made a breakout box with two 9 pin
D connectors and an MMJ socket along with the means to disconnect and
rewire as needed. I found that it works fine without DE9-4 being wired
to MMJ-6. In other words, using only the two data lines and ground.
Your mileage may vary, depending on how things are setup.

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DEC makes several adapters to adapt the MMJ to 9/25-pin D connectors.

The H8571-A and H8575-A are MMJ to DB25 (female) and are wired as follows:

MMJ DB25
 1 20
 2 2
 3 7
 4 7
 5 3
 6 6 & 8

Also pins 4 & 5 of the DB25 are tied together

I have used the above using a standard DEC BC16E cable to connect to
a PC 25-pin serial port to use the PC as a terminal.

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The H8571-C and H8571-F are MMJ to DB25 (male) and are wired as follows:

MMJ DB25
 1 6
 2 3
 3 7
 4 7
 5 2
 6 20

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The H8571-B an MMJ to DE9 (female) and is supposedly wired as follows:

MMJ DE9
 1 5
 2 2
 3 7
 4 7
 5 3
 6 6 & maybe 8 (depending on which DEC manual you read)

I have never seen one, so have not actually checked how it is wired.
It should be noted that this is not wired correctly for PC 9-pin
serial ports, besides the gender of the 9-pin connector is wrong.
DEC says that this is used for connecting to 9-pin printer ports.

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And here is a summary on how the various signal lines match up to
each other on the different connectors.

Term Term MMJ MMJ port on DEC
DB25 DE9 computer or Decserver

 20 4 1 --->-------------->----------------->--- 6
  2 3 2 --->-------------->----------------->--- 5
  7 5 3 ---------------------------------------- 4
  7 5 4 ---------------------------------------- 3
  3 2 5 ---<--------------<-----------------<--- 2
  6 6 6 ---<--------------<-----------------<--- 1

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Received on Thu Oct 19 2000 - 10:54:38 BST