|WWW.8BIT-MICRO.COM ONLINE VINTAGE COMPUTER MAGAZINE|
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
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.
FIRST WE WILL LOOK AT THE ACTUAL SERIAL PORT - The RS-232C
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:
You will notice that he older units, like the NEC, TRS-80 and vintage Tandy's all have 25 PIN RS-232C (Serial Ports)
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.
WHAT MUST HAPPEN FOR OUR DATA TO FLOW FROM A DTE TO DCE & VISA-VERSA
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):
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".
NULL MODEMS - DB25 PIN
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
Chances are the non-functioning NULL MODEM you bought at Radio Shack or Best Buy is a
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
HOW ABOUT OUR DB9 PIN NULL MODEMS
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
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.