MMJ Info

From: Jerome Fine <jhfine_at_idirect.com>
Date: Thu Oct 19 19:45:53 2000

>Mzthompson_at_aol.com wrote:

> [Snip]
> 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.

Jerome Fine replies:

I have one of the H8575-A, but have not yet done a continuity check to verify
the connections that you state. But I assume that you are correct. One of these
days I will attempt to verify that all of the above works with a VT320. Thank you.

Any suggestions on how to verify the above? Can a simple tester do this?

> 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

I have an H8571-F as well. When I pried apart the DB25 from the housing,
that was what I found.

> 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

Seems very reasonable. And it does work since I also connected up two
VT320 terminals using two cables and one female DB25 and one male
DB25 (i.e. one H8575-A and one H8571-F) after I had used just a
single BC16E cable.

However, I have two more VT320 terminals which I want to connect
and I don't have at least one H8571-F.

So here is the question. Since we have both found the cost of the H8575-A
and the H8571-F DB25 MMJ to DB25 connectors to be a bit high in the
price (i.e. more than we want to pay), would it be reasonable to cut the
cable in half and put an ordinary RJ11 phone connector on each cut
end to produce TWO cables? I also happen to have a couple of RJ11
to DB25 connectors (male) to which I could then connect the newly mated
RJ11 normal style. That way, I would not need to have the DEC MMJ
connectors. However, there is one minor problem - you note that both
pins 3 and 4 are connected to DB25 pin 7! Must that be done? The problem
is that the RJ11 end has all 6 pins connected to 6 individual male components
and it will be a hassle to attempt to connect both of pins 3 and 4 of the RJ11
to DB25 pin 7 - to say the least. May one be omitted and if so which one?

I think that this should be possible since I also happened to find another
MMJ DB25 female connector which has the "funny" DEC offset, but
without the DEC name. In this case, I was also to pry the DB25 off the
housing and found that pin 3 in the MMJ side was left hanging and not
connected to anything. In addition, on the DB25 side, pin 6 was not
connected to pin 8. I have not had a chance to test with both the DEC
(H8575-A) and the non-DEC DB25 female connectors with a VT320
at each end, so I am not sure if the non-DEC version can be used. But
since the non-DEC (i.e. without the DEC label) has the "funny" DEC
offset, I feel I can assume that it will probably work - in point of fact,
I might have used that one when I did the VT320 to VT320 test.

If my suggestion is possible, then it should be very easy to convert any
DEC BC16E cable into two cables with one end being used at the
VT320 and the other (after being fitted with a standard RJ11 connector)
to be used with a normal RJ11 to DB25 connector.

Just though I would ask for some advice and suggestion. Also, if anyone
has already done this, and it worked, then I would know it is going to be
OK. If there was something I should be aware of and it has already be
figured out, that would be helpful to know. Note that the VT320 terminals
will be used without a modem, so if all DEC parts were used, the BC16E
cable with an H8571-F would first be used to convert to a standard
DB25 male interface. Then a null modem cable would be used to connect
the DB25 male to the standard DB25 male on a DEC computer. The
key point is to attempt to become independent of the DEC "funny" offset
within the first cable connected to the VT320. The annoying part is that
I have a number of RJ11 to DB25 convertors without the "funny" DEC
offset plus a small amount of 6 and 8 wire cable that I can't use since
I first have to overcome the "funny" DEC offset.

If I don't receive a reply, I will still go ahead, but it may take a few weeks
or even a few months. I will post the results in case anyone is interested.
If I forget and anyone wants to shake me out of a "senior moment", please
inquire.

Sincerely yours,

Jerome Fine
Received on Thu Oct 19 2000 - 19:45:53 BST