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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 Name Abbreviation Pin No. Flow Direction
Protective Ground PROT. GND * 1 N/A
Transmitted Data TXD 2 ------>
Received Data RXD 3 <------
Request To Send RTS 4 ------>
Clear To Send CTS 5 <------
Data Set Ready DSR 6 <------
Signal Common SIG. GND * 7 <----->
Data Carrier Detect DCD 8 <------
Data Terminal Ready DTR 20 ------>
Ring Indicator RI 22 <------
* 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.Null Modem Without Handshaking - CLICK FOR LARGE IMAGE


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"Null Modem With FullHandshaking (FULL NULL MODEM) - CLICK  FOR LARGE IMAGE 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




DB9 PIN IBM style FULL NULL MODEM - CLICK FOR LARGE IMAGEOnce 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 Name Abbreviation Pin No. Flow Direction
Data Carrier Detect DCD 1 <------
Received Data RXD 2 <------
Transmitted Data TXD 3 ------>
Data Terminal Ready DTR 4 ------>
Signal Common SIG. GND * 5 <----->
Data Set Ready DSR 6 <------
Request To Send RTS 7 ------>
Clear To Send CTS 8 <------
Ring Indicator RI 9 <------


9PIN to 25PIN Serial Port Adaptor - CLICK FOR LARGE IMAGENow 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!




(c) 2004, 2005, Brian K. Hahn

 All Rights Reserved.