Tools and materials needed
- Wire cutter and stripper (a good lineman’s plier works well in my experience) Soldering iron Solder Screwdriver(s) Digital multimeter (optional)
- Two DB25 female connectors with hoods
or
- One DB25 female connector with hood and one DB9 female connector with hood
- 8 conductor wire (I use Category 5 networking cable)
You can obtain all the parts and tools necessary for this project at Radio Shack, but you could probably get them considerably cheaper elsewhere.
Phone Free
One of the most common questions that Apple II users ask today is, “How do I get data from one of my other computers onto my Apple II?” One of the simplest solutions is using a null modem connection. In this brief article we will discuss how to go about creating a very simple null modem adapter to go between an existing modem cable and an Intel compatible processor based computer (which we will term an “x86” for short).
I Thought Null Meant “None”
A null modem adapter is sometimes called a “modem eliminator”. In this case, it eliminates more than a modem—it eliminates two modems and two phone lines. Instead of having your Apple II attached to a modem which then calls your x86 machine via its modem via two phone lines, you simply attach the null modem adapter to your x86 machine on one side and to your Apple II’s modem cable on the other. You can then run telecommunications software on both computers and “talk” to each other, even exchanging files with protocol transfers, such as XModem, YModem, or ZModem. Additionally, those who use Linux or other UNIX type operating systems can use your Apple IIs as “smart terminals”, although discussing those is beyond the scope of this article.
The Nice Thing About Standards
. . . is that we have so many to choose from.
First off, nearly every modem cable I’ve seen has a DB25 male end. It’s possible—but extremely unlikely—that your cable has a different type of cable end. If it does, then you’ll have to figure the rest of this out on your own.
Secondly, there are, generally, two types of serial connectors on x86 machines—a DB9 female and a DB25 female. Again, it is possible your machine may have something else, but it is very unlikely. We will discuss null modem cables for both of these connectors.
Finally, while researching this arcticle and doing some experimenting on my own, I found no less than seven distinct ways to wire a null modem cable. While the Great Apple II Null Modem Shootout will have to wait, I have had great success using the connector I will share with you momentarily.
The Pin Outs
The DB25 connector was originally the one that was used with x86 machines, but of the 25 pins, only about 1/3 of them were used heavily. Those pins are:
| Pin number | Description |
|---|---|
| 2 | Transmit Data (TXD) |
| 3 | Receive Data (RXD) |
| 4 | Ready To Send (RTS) |
| 5 | Clear To Send (CTS) |
| 6 | Data Set Ready (DSR) |
| 7 | Signal Ground (GND) |
| 8 | Data Carrier Detect (DCD) |
| 20 | Data Terminal Ready (DTR) |
Looking at the female connectors which will connect to either the existing modem cable or the serial port on the x86 machine, the pins are numbered as follows (DB25 followed by DB9):
(ASCII art follows, for the visually impaired)
13 12 11 10 9 8 7 6 5 4 3 2 1 _______________________________________ \ o o o o o o o o o o o o o / \ o o o o o o o o o o o o / ---------------------------------- 25 24 23 22 21 20 19 18 17 16 15 14 5 4 3 2 1 ________________ \ o o o o o / \ o o o o / ------------ 9 8 7 6
(End ASCII art)
To make a null modem connection between the two computers, it’s necessary to connect receiving to transmitting lines as well as make the necessary handshaking connections. In this case, a DB25 to DB25 connection would look like this:
(ASCII art follows, for the visually impaired)
Female Female DB25 DB25 2<-----------------------------> 3 3<-----------------------------> 2 4<-----------------------------> 5 5<-----------------------------> 4 6<----------------------------->20 7<-----------------------------> 7 8<-----------------------------> 8 20<-----------------------------> 6
(End ASCII art)
The same cable but with a DB9 on one side would look like this:
(ASCII art follows, for the visually impaired)
Female Female DB25 DB 9 2<-----------------------------> 2 3<-----------------------------> 3 4<-----------------------------> 8 5<-----------------------------> 7 6<-----------------------------> 4 7<-----------------------------> 5 8<-----------------------------> 1 20<-----------------------------> 6
(End ASCII art)
Now that you have the information at hand, it’s time to start cutting wires and soldering. You can make the cable just about any length you want—I’ve had runs of fifty feet and more work fine. After you’re done, you can use a multimeter to test your work or just put the hoods on and connect up. Use a telecommunications program on each computer, set your baud rate and other settings to be the same on each side. Hit the return key a few times to see if data is getting through—if it is, be daring and try a protocol transfer on each end. If not, it might be time to do some more soldering.
Doing It In Hardware
If, after all of these instructions, you don’t feel comfortable doing this yourself, email me at rsuenaga@apple2.org and I’ll be happy to mail you one of these cables for a few dollars. It can help to keep your Apple II useful more than 20 years later.