Preparing Serial Cables

The cable must have at least one ground pin. Use pin 7 for ground, rather than pin 1, when possible, but you can also use both. Often people solder pin 1 to the cable's aluminum sheathing, or to the hood itself.

The cable must have lines for both CTS and RTS. These are pins 2 and 3 in most cables. This cable will be set up as a null modem cable, requiring pins 2 and 3 to be reversed.

The cable needs only a ground pin (1 or 7) and CTS/RTS (2 and 3) in a null modem configuration, for an XOn/XOff (software handshaking) cable. For this type of cabling, standard two-pair 22 gauge untwisted pair will work. However, it is better to use at least a four-pair cable (in case you snag or crimp wires or decide you need a second printer). Check each newly made cable with an Ohm meter to detect any problems in its construction (bad crimp, crossover of signals on pins, and so on).

A DSR/DTR (hardware handshaking) cable is essentially the same as XOn/XOff cable, but also requires the DSR and DTR pins (usually pins 6 and 20 on a DB25). These signals must be crossed in a null modem configuration.

Hewlett-Packard's PSERVER and NPRINTER programs query the status of pin 5. However, if you are printing directly, you will be using DOS, which generally queries pin 6. In order to use both of these pins, their signals must be "pulled high" (the voltage must be up for both pins). To pull these pins' signal high, we strongly recommend you tie pins 4-5-6-8 together at the printer end (but not the PC end). If you do not tie these pins together, you may find the printer mysteriously going off-line on the PSERVER console.

If a cable that you purchase has additional live pins, find out what they are by using an Ohm meter or similar device. If none of the additional pins are using the pins mentioned above (ground, CTS, RTS, DSR, and DTR), then there is a good chance the cable will work. However, the only sure way to find out if a cable works is to test it in the actual configuration. A serial-diagnostic device, such as a breakout box, can be useful if you will be working with serial cables frequently.