19 Oct
2000
19 Oct
'00
3:54 a.m.
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, 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
--------------- --------------- --------------- ---------------
---------------
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 BC16E 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
--------------- --------------- --------------- ---------------
---------------
19 Oct
19 Oct
12:45 p.m.
Mzthompson(a)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
5:12 p.m.
On Thu, 19 Oct 2000, Jerome Fine wrote:
[...]
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.
The following took less than 15 minutes (testing included):
Using a utility knife or small diagonal wire cutters, cut the retainer
clip portion from an RJ11. Just cut the thin springy portion.
Look carefully at the remaining portion of the RJ11. Name the side with
the contacts the 'bottom'. You will see that the 'top' has a ridge of
plastic at the front edge protruding above the flat surface of the 'top'.
The ridge is perhaps 1/16 inch, or less, high.
Look at the opening of the MMJ, with the MMJ oriented with the contacts on
the 'bottom'. You will see that the plastic ridge on the RJ11 will
conflict with the 'ceiling' of the MMJ, but only for a length of
approximately 1/16 inch on the right-hand corner of the ridge.
Using a small, not too coarse file (in a pinch, a nail file would probably
work), file down approximately 1/16 inch of the right-hand coner of the
ridge on the RJ11. As your file approaches the body of of the RJ11,
switch to cut - and - fit mode.
When the RJ11 fits into the MMJ, stop filing.
I did this to an RJ11 crimped to a 6 conductor lead. I achieved a snug
fit which, while not locked in place, will probably stay mated to the
HB571-C into which it was inserted for any reasonable use. The HB571-C is
covered in a sort of rubberized plastic shell which might provide more
friction than a harder plastic on, say, a terminal.
I tested the continuity of the connections for each of the 6 conductors in
the lead. Each conductor mapped to one of pins 2,3,6, or 20 of the
HB571-C; two conductors mapped to pin 7 (assuming I counted the *&^%@#%
pins right). Factory prepared 6 conductor leads with crimped RJ11
connectors are available in almost any length with the conductors in
either 'rolled' or not 'rolled' configuration.
I have not tried to use this to connect any equipment.
YMMV, but it should be a cheap way to defeat this particular DECism.
Bill
21 Oct
21 Oct
2:45 p.m.
William Fulmor wrote:
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.
The following took less
than 15 minutes (testing included): Using a utility knife or small diagonal wire cutters,
cut the retainer
clip portion from an RJ11. Just cut the thin springy portion.
Look carefully at the remaining portion of the RJ11. Name the side with
the contacts the 'bottom'. You will see that the 'top' has a ridge of
plastic at the front edge protruding above the flat surface of the 'top'.
The ridge is perhaps 1/16 inch, or less, high.
Look at the opening of the MMJ, with the MMJ oriented with the contacts on
the 'bottom'. You will see that the plastic ridge on the RJ11 will
conflict with the 'ceiling' of the MMJ, but only for a length of
approximately 1/16 inch on the right-hand corner of the ridge.
Using a small, not too coarse file (in a pinch, a nail file would probably
work), file down approximately 1/16 inch of the right-hand coner of the
ridge on the RJ11. As your file approaches the body of of the RJ11,
switch to cut - and - fit mode.
When the RJ11 fits into the MMJ, stop filing.
I did this to an RJ11 crimped to a 6 conductor lead. I achieved a snug
fit which, while not locked in place, will probably stay mated to the
HB571-C into which it was inserted for any reasonable use. The HB571-C is
covered in a sort of rubberized plastic shell which might provide more
friction than a harder plastic on, say, a terminal.
I tested the continuity of the connections for each of the 6 conductors in
the lead. Each conductor mapped to one of pins 2,3,6, or 20 of the
HB571-C; two conductors mapped to pin 7 (assuming I counted the *&^%@#%
pins right). Factory prepared 6 conductor leads with crimped RJ11
connectors are available in almost any length with the conductors in
either 'rolled' or not 'rolled' configuration.
I have not tried to use this to connect any equipment.
YMMV, but it should be a cheap way to defeat this particular DECism.
I think that the most satisfying part of the procedure is being able to
defeat the DECism. Seems like you enjoyed it as well.
Sincerely yours,
Jerome Fine
4:01 p.m.
On Sat, 21 Oct 2000, Jerome Fine wrote:
Jerome Fine replies:
I appreciate the suggestion made below, however, I believe that I
have located someone with some RJ11 blanks and a press. ASAP,
I will cut the current cable I have to test my concept. If that works,
I will not need to file anything - plus the connections should be as
stable and firm as with DEC hardware. Otherwise, I will try out
your method. Thank you.
Graybar Electric still stocks MMP's (AMP P/N 5-555237-200) and crimpers,
at least in some locations. But that will, of course require the
expenditure of $$, and they might not have an outlet near you. Also,
someone else on this list has (or had) access to a a source of crimpers -
he was kind enough to help me get a pair - but I've misplaced his name.
Bill
8829
days inactive
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4 comments
3 participants
participants (3)
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jhfine＠idirect.com -
Mzthompson＠aol.com -
wpfulmor＠dimensional.com