Making an MMJ to DE25-F adapter

From: Bob Lafleur <bob_lafleur_at_technologist.com>
Date: Wed May 29 22:04:48 2002

Tony,

Thank you for your detailed description of how to make a cable.

Unfortunately, my MMJ to DB25 adapter is made in such a way that I can't
see inside to tell which wires go to which pins. All I can do is assume
that the wire colors are the "standard" colors, i.e. green/red in the
center, black/yellow one pair out, etc.

While doing more research I also realized that I'm going to have to
connect multiple wires to the same pins. The wires that come out of the
MMJ jack have the "pins" right on them and they just fit into the holes
on the DB25 connector. So, hooking up multiple signals to the same pin
would be tough (I don't do soldering irons).

I found a source to sell me an adapter to do exactly what I need (this
adapter is what I thought I was getting in the first place, I didn't
realize I was getting a "make it yourself kit"). So I figured I might as
well order it and save myself a lot o f headaches. It should be here
tomorrow.

   - Bob

-----Original Message-----
From: cctalk-admin_at_classiccmp.org [mailto:cctalk-admin_at_classiccmp.org]
On Behalf Of Tony Duell
Sent: Wednesday, May 29, 2002 6:47 PM
To: cctalk_at_classiccmp.org
Subject: Re: Making an MMJ to DE25-F adapter


> I have an adapter that has MMJ female and a DE25-F connector. The

Argh!. I've spent years correcting 'DB9' to 'DE9', and now we get DE25
connectors ;-) There is no such thing. You are not going to pack 25 pins

into the E shell (DE9 and (H)DE15 are the 2 that exist). For once you
mean DB -- the standard RS232 connector is a DB25 (which is, I guess why

the PC-crowd call all D connectors 'DB').

Anyway. I'll assume you mean DB.

> adapter needs to be "made" (i.e. I've got wires coming out of it that
> plug into holes in the DE25-F connector). I want to make it so that
> I'll use an MMJ cable from the console port of my MicroVAX 3100 to the

> adapter, and plug the adapter into the 9-25 adapter cable that's
> connected to my PC Serial port.
>
> How do I wire it?

Well, the first thing we have to know is the wiring of that DE9 to DB25
PC cable. I think it's safe to assume that the DB25 is somewhat close to

RS232, but is the cable a null-modem or not. With those connectors it
should be (it would be used to connect 2 DTEs, both with male
connectors), but I'd not bet on it.

>
> I've got these color wires (not in any particular order)

And I can't beleive the colour code is necessarily standardised.

Here's what I would do. Start by looking at how the wires are connected
to the existing DB25-F in the MMJ adapter. If we assume that's standard
RS232 as well (again it could be either DTE or DCE), then we can do a
couple of quick tests.

Firstly, what pins are used on the existing adapter (and on the PC cable

if you can check it). I would be suprised if pins other than 2-8 and 20
were wired.

Then get one of those RS232 'quick checkers'. Tandy (Radio Shack) used
to
sell them. It's an in-line adapter with a DB25M at one end and a DB25F
at
the other. On the top are 7 bicolour LEDs. It passes all 25 pins
straight
through and also monitors 7 of them : 2 (TxD), 3 (RxD), 4 (RTS), 5
(CTS),
6 (DSR) 8 (CD) and 20 (DTR). Last time I bought one it cost about
\pounds
10 I think. Not too expensive, and I use mine a lot.

Conenct that to the existing MMJ adaptor (connected to a VAX or
whatever). See which LEDs light up. Those are the outputs. Wires that
are
connected, but which don't light the LED are inputs

Do the same with the PC cable.

We can now make a guess at how to connect it (at least for starters).

Pin 7 -- signal ground -- stays as it is. Move all the wires from that
pin on the DB25F on the MMJ adapter to pin 7 on the DB25-M

Pins 2 and 3 (the data lines). You need to connect an output to an
input.
So if pin 2 is an output on both the existing MMJ adapter and on the PC
cable, you need to swap over these 2 wires. Ditto if pin 3 is an output
on both existing devices. But if pin 2 is an output on one device and
pin
3 is an output on the other, just keep those wires on the same numbered
pins when you move them to the DB25-M

Pins 4 and 5 (RTS and CTS), do the same thing with. Keep them as a pair,

but link an output to an input

Pins 6 and 20 (DSR and DTR), again, do the same with those.

Of course you might have some weird combination of handshake pins used,
but if you tell us which pins are currently wired on the MMJ adapter,
and
whether they test as inputs or outputs, we might be able to make a
sensible guess.


> Which holes should I plug which colors into? Please describe how I
> should orient the connector when describing which holes to plug the
> wires into.

Looking at the wiring side of the DB25M (so also the face of the DB25F)

  1 o
      o 14
  2 o
      o 15
  3 o

 ....
 12 o
      o 25
 13 o

-tony
Received on Wed May 29 2002 - 22:04:48 BST