Technical Articles

A Null-Modem is a Null Modem Right? – Not Really


We take a closer look at why many people have trouble transferring files using a store bought Null Modem Adaptor.

– by: Brian K. Hahn

The number one email we get are from people is related to having trouble using the Tandy WP-2, Brother PowerNote or other vintage computers, for transferring files to a modern PC.  I must confess that I just did not understand why there are so many problems out there.  I have never had a single difficulty connecting may older machines to my Windows desktop.  For the Tandy Model WP-2 and Citizen CBM-10WP we offer WP2DOS as a download, and for the TRS-80 Models 100 and Tandy 102, Club 100 offers LapDOS II.  Every time I used these systems with Windows up to Version 98SE, and for LapDOS II MS-DOS and DOS environments up to Windows 98, I never had a hint of trouble. 

Until last month when I went out and bought a commercial Null-Modem Cable.  Then all the solutions stopped working.  So, I took out my trusty continuity tester and checked the pin-outs and discovered that not all Null Modems are created equal. The first hint I got was when my friend Rick Hansen from Club 100

insisted on his members using a Full Null Modem over a standard one.  Hey, somethings up here folks!  The bottom line is . . . . if you are using a vintage computer and for the most part that includes all TRS-80’s, Tandy, NEC, Olivetti, Kyocera, Citizen, Brother, Laser, DreamWriter, Amstrad to name the systems I have hands-on experience with, you MUST use a full null modem.

Now that we have said that, if you bought a Null Modem Cable or Null Modem Adaptor and you are not getting the system to work, AND, you have eliminated the possibility that your Serial Port on your desktop (MS-DOS / Windows) machine is NOT recognized and present. If you’re still having trouble, toss your Null Modem in the garbage.  Chances are it is NOT a FULL NULL MODEM. 

Here is another tip….  don’t bother talking to a Radio Shack salesman, Future Shop sales rep or any other retail person, they will just stand there and stare at you like they are watching hair grow.  They don’t know folks.  So sit back and read.  We will exercise the demon that has been confounding our communication problems.


The true RS-232C port is a 25-pin D-subminiature connector. The ‘male’ version of this connector is commonly known as a DB-25P, the ‘P’ standing for PIN.  It is the end with the pins, the female version is known as the DB-25S.  The ‘S’ standing for socket.  You get the picture.  Further, the standard RS-232C defines specific signal levels, where the “OFF” (MARKING) condition asserts a voltage of -3VDC to -25VDC, and the “ON” (SPACING) condition is the opposite, a voltage detected from +3VDC to +25VDC.  The true RS-232C has a specific set of PIN assignments.

They are as follows:

Signal NameAbbreviationPin No.Flow Direction
Protective GroundPROT. GND *1N/A
Transmitted DataTXD2——>
Received DataRXD3<——
Request To SendRTS4——>
Clear To SendCTS5<——
Data Set ReadyDSR6<——
Signal CommonSIG. GND *7<—–>
Data Carrier DetectDCD8<——
Data Terminal ReadyDTR20——>
Ring IndicatorRI22<——
* Non-Standard Abbreviation   

You will notice that he older units, like the NEC, TRS-80 and vintage Tandy’s all have 25 PIN RS-232C (Serial Ports) Before we go on, you must understand that the EIA (Electronics Industry Alliance) and the CCITT (Comite Consultatif Internationale de Telegraphie), two bodies that set communication standards in industry, have defined two types of devices.  They are the DTE or Data Terminal Device, (your Computer), and the DCE, the Data Communications Equipment (a Modem).  The above table refers to the DTE, the personal computer RS-232C.  For a complete list of definitions on the PIN definitions click here.


Both the DTE (Computer) and DCE (Modem) must have a handshake, that must be “flow compatible”.  That is when the Computer sends data, the modem must be open to receive it and visa-versa.  This is an over simplified explanation but for our purposes it makes sense.  A more technical definition is as follows:

For our data to flow from DTE to DCE (computer to modem):
    —– > The DTE must assert DTR and RTS
    —– > The DCE must assert DSR, DCD, and CTS

For our data to flow from DCE to DTE (modem to computer):
   < —– The DTE must assert DTR
   < —– The DCE must assert DSR and DCD

What we have covered so far are the standards by which our vintage computers hold onto, but not all RS-232C interfaces are created equal.  Already you know that many of these ports are a DB9P (9 PIN) configuration, where the PIN assignments can only have 9 variables.  This is evident in the Tandy WP-2 and all the newer computers.  In fact, you are probably trying to hook up your vintage micro to your Windows based computer via a 9PIN serial port. 

This is just one of the problems.

Many manufacturers do not support all ten PINS that we have listed in our table.  Many just support seven PINS and that requires a special NULL MODEM configuration.  Further, other manufacturers put special signals on some of the PINS.  With all these differences, and there are other examples, chances of you finding a “off the shelf” solution are limited.  But it can be done, the answer is to locate or better, make a Full Null Modem Adaptor. To built one see our archived article, “Transfer Points”.


To get two devices to transfer information between them using a serial cable, we must fool those DTE devices (computer to computer) to believe they are connected to a DCE (Modem).  Simple right.  Actually it is, we do that by using the NULL MODEM.

Chances are the non-functioning NULL MODEM you bought at Radio Shack or Best Buy is a what we call a SIMPLE NULL MODEM.  This is the “retail” standard that is designed for connecting two DTE devices that do not require HANDSHAKING.  This is accomplished by swapping PINS 2 and 3 between each end.  This is where many of us have gone wrong, and this is why a person fails to get WP2DOS or LAPDOS to work with there computers. 

A FULL NULL MODEM is different in that it crosses PINS 4 to 5 at each end, plus PINS 6 to 22

at each end when using a DB25 connector.  This provides our computers (DTE) full handshaking capability. 


Once we understand the difference between the two basic NULL MODEMS the leap to the 9 PIN

is not that mysterious.  This configuration was first introduced by IBM and embraced by many

including Tandy and Citizen.  The 9PIN configuration was desirable for many reasons, the

foremost was the smaller size of footprint for PC Board mounting.

The following table shows the PIN assignments on a DB9 PIN RS232C connector, as defined by


Signal NameAbbreviationPin No.Flow Direction
Data Carrier DetectDCD1<——
Received DataRXD2<——
Transmitted DataTXD3——>
Data Terminal ReadyDTR4——>
Signal CommonSIG. GND *5<—–>
Data Set ReadyDSR6<——
Request To SendRTS7——>
Clear To SendCTS8<——
Ring IndicatorRI9<——

Now that we have our DB25C/S and DB9C/S Null Modems defined, we now must have a way to convert from a 25 PIN connection to the smaller 9 PIN connection on our new machines.  This is simply done by using a 9 PIN to 25 PIN adaptor.  The good news is that this is where the retail stores have it right.  You can walk right in and get one at any Radio Shack or electronics store even a Wal-Mart and you will have a correct connector.  But for the purists out there the thumbnail to the right has a diagram showing you the PIN assignments of a 9 to 25 PIN adaptor.


Obtain or built your own “FULL NULL MODEM” adaptor in conjunction with a standard serial cable.  If you have this in your possession, then networking your old Tandy WP2 to a new Personal Computer and running the WP2DOS software or even a terminal program to transfer your files will be simple.  This does work, and you do not need a fancy IR port or a PHD to enjoy flawless file transfers.  Download the software and get the proper tools.  Then visit Google’s Newsgroups and read. You will find that many out there are just not finding the answers.  Who knows maybe you to can be the next computer guru.  Happy computing!