Null Modem Adapters

A while back, I had a bunch of null-modem adapters made up. They have worked pretty well for a wide variety of applications, and a number of people have asked me about them. I wrote up a description of them to send out to a couple of mailing lists, and that description has been sitting as an unlinked text file in my http directory for a year or so. Nevertheless, people keep finding it and I get the occasional question about it, so I figured I'd put it here as a web page to make it even easier.

I wish to emphasize that this is far from the only way to do this. This is just the way that I did it, and it's been shown to work for several people. Use it or ignore it as you wish. However, I do enjoy hearing from people who have built them.

NB: If anyone decides to sell these as a commercial product, please let me know and I'll include a link here to your company's website

Requisition for Full-Handshaking Modular Serial Adapters for Use in the Simple Construction of RS-232 Null Modem Cables.

There are four custom modular serial adapters specified in this requisition. The intent is to have the ability to create full-handshake RS-232 null modem cables by using pairs of these adapters and four-pair straight-through Cat 5 patch cables (the cables will be procured seperately). As designed, any of the following combinations should work to provide the full-handshake null modem capability:

 Type1 <-> Type2
 Type1 <-> Type4
 Type3 <-> Type2
 Type3 <-> Type4

With the <-> representing a four-pair cat 5 straight-through patch cable terminated in RJ-45 modular plugs. In the following, "shell" means the metal shell of the D-sub contact housing, and the attachment to the shell must be soldered. All-plastic contact housings are not acceptable in this application.

Type 1

These modular adapters must have the following pinning:

  RJ45         DB25 (Female)
  1 ---------- 1 -- shell
  2 ---------- 2
  3 ---------- 3
  4 ---------- 4
  5 ---------- 5
  6 ---------- 6 -- 8
  7 ---------- 7
  8 ---------- 20

The representation "6 ---------- 6 -- 8" is meant to indicate that pins 6 and 8 on the DB25 must be tied together and jointly connected to pin 6 of the RJ-45.

Type 2

Pinning for this adapter is as follows

  RJ45        DB25 (Female)
  1 ---------- 1 -- shell
  2 ---------- 3
  3 ---------- 2
  4 ---------- 5
  5 ---------- 4
  6 ---------- 20
  7 ---------- 7
  8 ---------- 6 -- 8

The representation "8 ---------- 6 -- 8" is meant to indicate that pins 6 and 8 on the DB25 must be tied together and jointly connected to pin 8 of the RJ-45.

Type 3

These modular adapters must have the following pinning:

  RJ45        DB9 (Female)
  1 ---------- shell
  2 ---------- 3
  3 ---------- 2
  4 ---------- 7
  5 ---------- 8
  6 ---------- 6 -- 1
  7 ---------- 5
  8 ---------- 4

The representation "6 ---------- 6 -- 1" is meant to indicate that pins 6 and 1 on the DB9 must be tied together and jointly connected to pin 6 of the RJ-45.

Type 4

These modular adapters must have the following pinning:

  RJ45        DB9 (Female)
  1 ---------- shell
  2 ---------- 2
  3 ---------- 3
  4 ---------- 8
  5 ---------- 7
  6 ---------- 4
  7 ---------- 5
  8 ---------- 6 -- 1

The representation "8 ---------- 6 -- 1" is meant to indicate that pins 6 and 1 on the DB9 must be tied together and jointly connected to pin 8 of the RJ-45.

Notes

Following is an email about this that I sent to several mailing lists:

From: Bob Drzyzgula 
To: PICLIST@MITVMA.MIT.EDU, linux-smp@vger.rutgers.edu, sparclinux@vger.rutgers.edu, beowulf@beowulf.gsfc.nasa.gov
Cc: bob@drzyzgula.org
Subject: FYI: Spec for serial null-modem modular adapters
Reply-To: Bob Drzyzgula 
X-Editor: vim
X-Operating-System: Redhat Linux 5.0

Hi. This is an FYI not meant for discussion (unless y'all 
want). I'd just been asked for about the fifth time for the
specifications for some serial null-modem adapters that 
I had made up a few months ago, so I figured I'd write
it up in a form that can be used as the starting point
for a requisition or purchase order, and put it where
people can find it. Rather than post it to all these 
lists, I've put it on my pitiful website at:

   http://www.eskimo.com/~bob/serial-spec.txt

I'm just letting y'all know in case you might find
it useful. I had 200 of each of the four types made up
early this year for my office, and generally they've been
solid and reliable; I've not come across a standard, basic
null-modem application that these won't work for (they work
well on a Sun ttya console) and they do generally work in
many straight-through applications as well. We got ours 
made by NuData (part of fill-in-the-blank warehouse at
http://www.warehouse.com), but pretty much any custom
cable shop  should be able to make them. In moderate
volumes, they can come out about the same or less than
pin-it-yourself parts. I suspect that others have come up
with similar designs, some of which may have significant
advantages over these. But these have worked well for us.
BTW, the only commonly used asynch signal that isn't
handled by these is RI, and both signal ground and
frame ground are provided. TC and RC are not pinned,
so these won't work for a synchronous line.

Let me know if you find something wrong or have suggestions
or problems with it.

Follow-up

As I mentioned, I occasionally get questions about this, and sometimes these do a good job of pointing out the shortcomings in my description. Here's the result of one such exchange, which may be helpful to others as well. Mike kindly consented to having his email and diagram included on this page.

From: Mike Ingram [mailto:mike.ingram@(obscured)] 
Sent: Tuesday, June 06, 2000 3:35 PM 
To: bob@drzyzgula.org 
Subject: question about null modem adapters 

Hey Bob, I found your response to someone's question about null
modem adapters (http://www.eskimo.com/~bob/serial-spec.txt)
and I'm confused about something and wondering if you have a
couple minutes to help me out.  I tried to buy a DB9F > RJ45
Null Modem Adapter at the computer/electronics store, but they
told me they don't sell them that way anymore, so I had to buy
simply a "do-it-yourself" adapter, and they didn't have the
specs for wiring them.  That's where you came into the picture.

I have figured out the numbering sequence of the wires coming
from the rj45 side, and I have inserted them into the db9 female
side, except for the remaining 2 wires which I'm confused about.

I am using your "Type 3" guidelines and have inserted all the
wires except for: orange (1) and green (6) (these are both
coming FROM the rj45 side)

You say to wire the DB9 1 and 6 wires together, but I must not
have exactly what you're talking about because my DB9 side only
has holes, and no wires.  So I have OPEN holes in the DB9F side,
holes 1, 6 and 9.  And to reiterate, wires 1 and 6 from the
rj45 side unused so far.

So my stupid question is what do I do with the two remaining
wires and 3 remaining holes?  Attached is a better description
of exactly what I'm talking about.  Thanks so much for your time.

Mike's Picture

Here is my response

Mike,

Glad you found my description helpful. I'd almost forgotten
about it being out there.

Basically, the problem you're having is that you can't
make my design without either some solder or a crimp tool,
preferably both. The DIY adapters they sell have some
limitations when it comes to doing this kind of thing.

Re RJ45 pin 6:

If you have a crimp tool and some contacts, what you
do is take the line from RJ45 pin 6 and trim off the
contact, leaving as much of the wire as possible. Strip
about 1/8" of the insulation off the end of the remaining
wire. Then take another, maybe 1 to 1.5" wire and strip
1/8" of insulation off both ends. Crimp a contact on
one end of the short, free wire. Then take the free end
of the pin 6 wire and the second end of the short wire,
and crimp them both into the same contact.  Then put one
contact in hole 6 of the DB9 and the other contact in
hole 1. If you *don't* have a crimp tool and contacts,
what you will need to do is to sacrifice a second
adapter, by snipping one of the wires off at the RJ45
end. Then on the keeper adapter, snip the pin 6 wire
in half, and strip a bit from the three loose ends
you now have (two from the pin 6 wire -- one to the
contact and one to the RJ45 -- and one from the wire
you stole from the other adapter). Then you will want
to solder the three ends together and wrap them with
some electrical tape or heat-shrink tubing (if you use
heat shrink, remember to put it on the RJ45 end of the
wire *before* soldering and then slide it down over the
solder joint and shrink. I always forget to do that kind
of thing). Then, you can put the two contacts coming
from RJ45 pin 6 into holes 6 and 1 of the DB9. If you
have a bunch of these to do, I guess you'll be able to
get eight of these extra wires out of each sacrificial
connector.

Re RJ45 pin 1:

This pin carries the frame ground. What you need to do is
to snip off the contact (actually, I suppose, this could
be a source for an extra contact for the pin 6 thing,
although the remaining wire length could wind up being a
problem) and strip off some insulation. Then scratch or
sand off some of the plating on the metal shell of the
DB9 contact housing. Tin the end of the pin 1 wire and
solder it to the DB9 shell. It sounds crude, but that
is in fact the official way to do it -- even the ones I
had made professionally were done that way. A DB25 has an
extra pin for the frame ground (even though the shell is
still supposed to be connected to frame ground as well,
so my design connects both; thus it shouldn't matter if
your computer or whatever isn't properly grounded in both
places if you use my design), but on the DB9 there's not
enough pins and they use the shell as a contact.

When all is said and done, you could probably get away
with not doing this grounded-shell part at all and just
going ahead and using the RJ pin 1 wire for the DB pin 1-6
jumper. All the pin 1 to shell connection does is connect
the frame grounds of the two devices, and if both devices
are both properly grounded to the same reference, there
should be no potential difference between the two shells
anyway. Also, if you look at one of the devices you are
connecting and find that the metal shell of the male DB9 is
connected to nothing, like for example it was mounted in a
plastic case and there was no wire deliberately connecting
the shell to the device's ground, then the frame ground
isn't going to do anything for you anyway. I would expect
that a fairly small number of null-modem cables that are
ever built do this correctly.

As for DB9 pin 9, you don't need it. That is used for the
RI (ring indicator) signal from a modem to a computer, and
if you're building a null modem adapter, well....

Actually, this does point out a caution. In very many
cases, you can use a pair of even-type (2 and 2, 2 and 4 or
4 and 4) connectors or a pair of odd-type (1 and 1, 1 and
3 or 3 and 3) to form a straight-through cable. However,
you can't do this if you actually need the handshaking
signals to work correctly  -- mostly this works for your
basic RX/TX/SG cable -- and you can't do it to connect a
modem because RI isn't supported.

Actually, I think I might paste the text of this message
into that document, so let me know if something I said
doesn't make sense, or if you'd mind me including your
question.

Hope this helps,
--Bob

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