20 Oct
2000
20 Oct
'00
5 p.m.
On Thu, 19 Oct 2000, Jerome Fine wrote:
verify
I'm not one to say I told you so, but..... ;-)
I assume you meant "I don't have more than one H8571-F"
If you need, I got a couple spares.
[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 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.
These adapters are listed in a few DEC manuals, so that is the primary
source of the info. In most cases I have verified the info as well.
Any suggestions on how to verify the above? Can a
simple tester do this?
A simple VOM. The hard part is staying connected to the pins. To make
that a little easier I use some adapters I picked up years ago. These
are banana jacks to D connector pins & sockets. You can roll your own,
but they are made by Pomona Electronics, p/n's 3560 & 3561.
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'll use the standard engineering answer. It depends. That is to say it
depends on what you are trying to do. There are times where you only need
certain lines, and there are times when you hook up all lines and it still
won't work.
I have had good luck in using just the transmit and receive data leads
(MMJ 2 & 5 to DB25 pins 2 & 3, and of course ground) to use for example a
PC as a terminal hooked to a DEC machine. If your going to hook a DEC
machine up to a modem then you are going to need the control lines as well.
On the other end, I attempted to use a Compaq portable one time as a
terminal and it did not work. I figured the serial port was bad.
Later when I tried again, I found that the port was fine, but still
would not interface with a DEC machine. I decided the reason had to
be due to the voltage levels. A standard serial (if it exists) as defined
by RS232C should have a voltage swing from +15 (0) volts to -15 volts (1).
Granted you will find it implemented at +/- 12v, +/- 9v, etc. These all
work as the spec defines the threshold as 5v, between +5 and -5 is undefined
and considered the transition zone. Now look at DEC, which uses DEC-423 which
I assume is a derivative of RS423. The voltage swing for 423 is +6 to -6
volts
with the threshold being 200 mv. I checked the DEC machine and sure enough
it was running at 6v. I concluded the the Compaq was possibly designed
with a higher threshold volatge, and was not going to recognize the 6v
coming from the DEC machine.
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.
That's OK. Pin 3 & 4 are both ground on the MMJ. I'll bet MMJ-4 is
connected to DB25-7, so there is the ground connection.
In addition, on the DB25 side, pin 6 was not
connected to pin 8.
That should be OK, 6 - DSR Data Set Ready, 8 - Data Carrier Detect. Here
again we are getting into modem control lines and may not be needed depending
on what you are trying to connect.
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.
Tongue in cheek. Oh my, he's gonna cut a BC16E cable into two pieces.
What's next, testing chain saws on Mac's, or removing spare parts from
heavy metal VAXen's with a propane torch. ;)
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.
Jerome, it sounds like a plan. But I have to ask, why the mindset about
the "funny" DEC offset? Is my perspective askew? By that I mean am I
living near the land of plenty? I have gleened one building after a computer
outfit moved out and hauled cables out in multiple 30 gallon garbage bags.
Not all were MMJ cables, but got enough to do my thing.
OTOH I understand, I have concocted my share of workarounds over the years.
Mike