Dec 142017
How to construct a null modem cable, used to connect 2 PC's by their serial ports.
File NULBABLE.ZIP from The Programmer’s Corner in
Category Tutorials + Patches
How to construct a null modem cable, used to connect 2 PC’s by their serial ports.
File Name File Size Zip Size Zip Type
NULCABLE.TXT 10007 3661 deflated

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Contents of the NULCABLE.TXT file

Here's how to construct a null modem cable, used to connect 2 PC's by their
serial ports. This allows you to transfer files from one PC to another at up
to 115,200 baud, providing a fast and easy way to transfer files which are
too large to fit on diskettes, or solving the problem of transfers when 2
PC's have incompatible disk drives. A suitable cable can be purchased
already made, or you can make one yourself. In addition, you will need some
type of software to manage the job. There are commercial packages out there
(LapLink and FastLynx are 2 examples), or you can get the appropriate
software through shareware sources, or free one from other sources. One such
program is PC Magazine's ZCOPY, available from their sources, or as printed
in PC Magazine, February 28, 1989 issue.

It's probably about the same price, and less fuss to buy the proper cable
from a decent computer shop. If your like me, however, you like making your

All the necessary parts are available at Radio Shack or a similar store. I
paid about $13 for all that was needed at Radio Shack. Additionally, you
will need:

Electrical or similar tape. Helps.
Small blade or phillips screwdriver (for the connector you buy)
Tweezers or forceps
Sharp knife or wire stripper
Soldering iron & solder (if using the solder type)
Connector Crimping Tool (for AMP brand connectors)
Wire cutters

Eight or nine conductor cable works well, and allows for addition of
connections if they should be needed in the future. To use the crimp type
connections, you need the tool to crimp the pins onto the wires. If you plan
on making several cable in your lifetime, then the tool is well worth it.
Otherwise, you're probably better off paying someone else to make it. Scout
your local electronics store. I've had good results with the connectors from
made by AMP, sold by your finer electronics stores. Also, Belden cable is
some of the best stuff to buy, if your retailer sells it.

If the computers you are connecting have 9 pin serial connectors, you need
the female (with holes) connectors on both ends of your cable. (IBM's and
compatibles have male connectors for their 9 pin serial ports). If yours are
female, make sure the connector is not a video connector instead of a serial
port connector. 25 pin COM ports on IBM's and clones are typically male,
also. Whether you make a cable with 9 pin or 25 pin connectors (or one at
each end), pre-made adapters can be purchased to convert from 9 pin to 25
pin, and vice versa. I made all mine with 9 pin ends, and use 9 pin to 25
pin adapters with good results.

TIP: Buy enough cable! As long as you're making it, make a length of about 8
or 10 feet. I've made cables, when using shield, about 35' in length
with no problems. Better a little too long than too short. Avoid excess length,

TIP: If you end up with Radio Shack parts, wrap electrical tape around the
cable at each end to fit underneath the connector. The connectors and cable
at Radio Shack don't fit real snug together, allowing the cable to slip out
and put stress on the electrical connections. You'll see what I mean when
you are closing the connector together.

TIP: If you are using Radio Shack cable, try to use the shield (the braided
wire around the outside of the cable as the ground connection. This task is
made easier if you use Belden cable - it comes with a wire that runs along
the shield, which is foil instead of that crummy braided wire. By using the
shielded part of the cable as the ground connection, it helps protect from
electrical noise. This is more important with longer cables.
The following is part of the docs for FastLynx:

-------------------------- Cable Specifications ----------------------------
Following is a description of the pin connections for a FastLynx 7-wire
serial cable. The cable is a 4-headed cable with a 9-pin and 25-pin female
connector on both ends. The cable is wired as follows:

9 pin 25 pin 25 pin 9 pin
----- ------ ------ -----
pin 5 pin 7 <----> pin 7 pin 5 (Ground - Ground)

pin 3 pin 2 <----> pin 3 pin 2 (Transmit - Receive)
pin 7 pin 4 <----> pin 5 pin 8 (RTS - CTS)
pin 6 pin 6 <----> pin 20 pin 4 (DSR - DTR)

pin 2 pin 3 <----> pin 2 pin 3 (Receive - Transmit)
pin 8 pin 5 <----> pin 4 pin 7 (CTS - RTS)
pin 4 pin 20 <----> pin 6 pin 6 (DTR - DSR)

The ground wire is connected to the same pin on both ends. The last three
wires are a reverse of the prior three.

Following is a description of the pin connections for a FastLynx parallel
cable. The cable has a male DB25 connector at both ends.

25 pin 25 pin
------ ------
pin 2 <----> pin 15
pin 3 <----> pin 13
pin 4 <----> pin 12
pin 5 <----> pin 10
pin 6 <----> pin 11

pin 15 <----> pin 2
pin 13 <----> pin 3
pin 12 <----> pin 4
pin 10 <----> pin 5
pin 11 <----> pin 6

pin 25 <----> pin 25

The second set of 5 wires is the reverse of the first set.

The following cable will allow transfers using LapLink III. However, it
doesn't support the feature of installing the software from the remote. The
FastLynx cable above does work with all the features of FastLynx. The
following cable merely transmits and receives data. It cheats by jumping
connections at each end to trick the computer into thinking it's connected to
another computer. The FastLynx cable above allows the 2 PC's to actually
communicate. However, I haven't gotten LapLink III to install software from
remote with FastLynx's cable, either. FastLynx does it just fine when using
a FastLynx type cable. Update 11/13/90: Uncle Heavy uploaded diagrams to make
a true Laplink III cable. The instructions are identical to the FastLynx cable.


Connector 1 Connector 2
----------- V -----------
Transmit Data 2 <================\ /-------------------> 2 Transmit Data
Receive Data 3 <-----------------/\====================> 3 Receive Data

RTS 4 < > 4 RTS

CTS 5 < > 5 CTS

DSR 6 < > 6 DSR

Ground 7 <-----------------------------------> 7 Ground

CD 8 < > 8 CD
See See
DTR 20 < ** ** > 20 DTR


*Connect pin #2 of one connector to pin #3 at the other end. This is known as
a "pin 2 to 3 crossover". That way one computer receives what the other is

At each end, connect pins #4, #5, & #8 together.

Also at each end, connect pins #6 & #20.
** If you are using a nine pin connector, this connection is not needed as
there is no pin #20. A connection to pin #6 is not needed.

TIP: Before you get too far, cut off about 3/4 inch of cable off one end of
your length of cable. Then, strip the insulation and foil from this piece or
use tweezers or forceps to remove the 9 wires from inside. Strip the
insulation off both ends of 4 of these wires, 6 if making a 25 pin connector
cable. These short pieces of wire will be needed to make the jumpers at each
connector. Twist one end of each of 2 wires together, and solder them both
pin #5. Then one wire can go to pin #4, and the other to pin #8 as in the

In case you want to know:
TD = Transmit Data
RD = Receive Data
RTS = Request To Send
CTS = Clear To Send
DSR = Data Set Ready
CD = Carrier Detect
DTR = Data Terminal Ready

I received instructions from a BBS one day on how to construct a null modem
cable, and decided to add some comments of my own. Then, I stumbled upon the
FastLynx documentation. I've made their cable, and it works quite well (at
least the serial cable does. I think I made the parallel cable a little
long. Back to the crimping tool!). So, this text is a culmination of all
three - the original file, my comments and ideas, and part of the FastLynx
documentation. -"Joe's Cat"


Following is part of the Laplink III documentation uploaded by "Uncle Heavy":

* 2 -------- 32
* 3 -------- 13
* 4 -------- 12
* 5 -------- 10
* 6 -------- 11
* 10 --------- 5
* 11 --------- 6
* 12 --------- 4
* 13 --------- 3
* 15 --------- 2
* 25 -------- 30

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