Making a cable to connect a PC modem to a Mac

Most PC modems have an RS-232 interface, which uses a 25-pin DB-25 connector or a 9-pin DB-9 connector. All Macs until the new G3 generation (which has no more "ordinary" serial ports) have an RS-422 interface via a DIN-8 connector, also called GeoPort connector. Luckily, both interfaces are compatible in a certain way. The only thing you need to do, is connect the right pins together. I once needed to connect a PC modem to a Mac and after having searched endlessly on the Internet, I found 5 different connection schemes. I finally blew up the modem's drivers because the scheme I thought to be the correct one, was completely wrong. So it was about time to provide some correct information about this, and here it is!

din8-db9The scheme at the left should be self-explanatory. As you can see, it is the scheme for two male connectors. A cable with 5 wires will do, although you actually should also connect the shield of both connectors together with a 6th wire. But since most modems connect the GND with the shield, you can simply connect the GND with the shield too at one of the connectors (not at both), so you only need 5 wires. Don't forget to use a shielded cable if it's longer than a few inches.

If for some reason, you need to use a female DB-9 connector, you just have to flip all connections horizontally, or in other words: the pins, or rather holes, are counted from right to left. If you need to use a DB-25 connector, you can either make a cable with a DB-9 connector and use an adaptor, or use a DB-25 connector right away. You can use the following table, which is nothing else than a textual representation of the images, to determine which pins should be connected:

RS-422 signal DIN-8 DB-9 DB-25 RS-232 signal
HSKo 1 4,7 20,4 DTR,RTS
HSKi 2 6 6 DSR
TxD- 3 3 2 TxD
Gnd 4 5 7 Gnd
RxD- 5 2 3 RxD
RxD+ 8 5 7 Gnd

A good advice: test the cable before using it, to see if you didn' accidentally swap two pins, or connect two pins together. Although most output drivers for RS-232 and 422 signals can withstand a short-circuit for a few seconds, it may be too late until you notice there is something wrong! If you connect two 'output' signals together, you will get a situation where two output drivers are "fighting" against each other: the one wants to drive the signal low while the other wants it high. This contest ends when one of the drivers burns out... Luckily, in my case the Mac was the winner and the dead chip in the modem could be replaced fairly easily.

Alexander Thomas, 02/2001