The BayCom modem uses the RS-232 lines in a quite different way from the way the RS-232 designers intended it to be used. The main reason for this is that AX.25 requires synchronous operation, which is not supported by the RS-232's 8250 interface chip (and compatibles): they expect asynchronous data with start and stop bits.
As a result, we cannot use the RXD and TXD lines of the RS-232 interface. For BayCom modems, the RS-232 lines are used as follows:
| RS-232 pin | RS-232 name | modem use |
|---|---|---|
| 3 | TXD | Power supply only |
| 4 | DTR | TX Data |
| 5 | GND | GND |
| 7 | RTS | PTT |
| 8 | CTS | RX Data |
For programming reasons, pin 3 outputs an irregular clock that may be used as the modem's power supply. Further power may also be obtained from RTS (on "high" when the modem is transmitting) and DTR (on "high" while the modem receives).
