Because of the size and length of the HP serial cable and the need for a multitude of adapters (9-to-25, 25-to-9, male-to-male, female-to-female, null modem, etc... ), I quickly decided that there must be a better way to solve the connectivity puzzle.
I cut the 9-pin end off the HP cable the 2nd night I had it! The HP cable had three wires (Red/ White/Yellow) and a ground wrap-wire wrapped around the others used to shield them from interference and act as a ground wire. (You can purchase an "Unterminated" 95LX serial cable, with the end already "cut off," from EduCALC for $12.95. See Order Information box, next page.)
I used a continuity checker to determine which signals were connected to which wires and then connected it to a four-prong RJ-11 modular phone adapter on the cut end of the HP cable. I then connected the new modular end of the HP cable to a 9-pin communications connector (RJ-11 to male 9-pin adapter) and attached it to a Macronix Fax/Modem. I was operational! I also have an RJ-11 to 25-pin male adapter for use with a Practical Peripherals Pocket Modem.
In the months I've been using this set up I've encountered no problems connecting to my home/office PC (386/33 megahertz IBM clone), a notebook PC, and various modems. I've successfully connected to a Macronix Fax/ Modem. I've also connected to the Practical Peripherals, Inc. Pocket Modem (PPI/ PM) using a non-approved modification to the PPI battery pack.
Connectivity Cable Modified to Use Phone Jack: Graphic
If you feel comfortable with a few cut wires, it really isn't a large task to accomplish the above. (More information on modem modification can be found in PM2400.95(ON DISK ICON) or TINYMO.95(ON DISK ICON), two text files in Library 6 of the HP Handheld forum of CompuServe. Modification of the GVC Mini Modem is described in GVCMOD.95(ON DISK ICON) and on page 2 of the Fall, 1991 issue of The HP Palmtop Paper.)
Creating a Cable to Connect to a Modem or PC
Every piece of hardware with a serial port needs the following minimum configuration in order to send and receive data: A wire for transmitting data; a wire for receiving data; and a third wire for ground to establish a marking point for positive and negative signals. The HP serial cable has a white transmit data wire, a yellow receive data wire, and a red ground wire.
Different modems have different types of serial ports; either a 9-pin or 25-pin male or female port. The HP 95LX Serial Cable already has a 9-pin female connector on one end, with the white transmit wire connect to pin 3, the yellow receive wire to pin 2, and the red ground wire connected to pin 7.
If you wanted to make a custom cable for your modem, you would have to cut that off and attach the appropriate serial connector to that end. If you were to attach a 9-pin male connector, you would attach the wires to the same pins as with the female 9-pin connector. If you were to attach the cable to a 25-pin RS-232 connector, you would solder the white transmit wire to pin 2, the yellow receive wire to pin 3 and the red ground wire to pin 7. (Don't confuse the insulated wires with the ground wrap, an uninsulated copper wire in the cable.)
Use the above instructions if you want to construct a cable to connect the 95LX to a MODEM. If you want a custom cable to connect the 95LX to another PC, you'll need to reverse the wires connected to pins 2 and 3, on both the 25-pin, or 9-pin connector.
You reverse wires when connecting to a PC because you want to connect the Transmit data pin of one computer to the Receive data pin on the second computer. When you send your message through a modem, the phone company conveniently crosses the send and receive data through its network. This is necessary even for normal phone communication so that your voice, which is transmitted from the mouthpiece on one end, is HEARD on the earpiece on the other side! When you connect to a local PC you have to cross these wires yourself!
I hope this gives you the necessary information to go forward, but once again, BEWARE! DO THIS AT YOUR OWN RISK! You may invalidate your warranty on any products you may connect to this modified cable.