[SunRescue] Serial Cables for Vt420 terminal

[BSD Bob the old greybeard BSD freak]

> I happened across some VT420's and VT320's at an auction ( Got 30 for 5$
> total, but I only had room to take 10 home) but to my dismay, none have
> the darned MMJ-DB25 cables. Does anyone happen to have any of these or
> know where I can find some?
> Thanks!

Neat, I get them for a buck each at the local moo U surplus.

Anyway.... The VMS faq has the pinouts.  But, for VAXentoyz, I go
by my local cable mongers shoppe, and he makes them up for me at
about 2 bucks each mmj/mmj for a 6 footer.  It takes a special
crimper for them, and by stroke of luck they had one.  Below is
the pinout info that is floating about the net.....

Bob


Info on the MMJ connector found on various DEC terminals & computers.
--------------- --------------- --------------- --------------- ---------------
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

--------------- --------------- --------------- --------------- ---------------
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

--------------- --------------- --------------- --------------- ---------------
DEC's part number for the cable is BC16E-xx, I assume the xx may be
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

--------------- --------------- --------------- --------------- ---------------
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.

--------------- --------------- --------------- --------------- ---------------
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 MMP cable to connect to 
a PC 25-pin serial port to use the PC as a terminal.

---------------
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

---------------
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.

--------------- --------------- --------------- --------------- ---------------
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

--------------- --------------- --------------- --------------- ---------------