Use the Serial Line Sniffer program:
Consider the following scenario:
All connections are set as 9600 N81.
Device (A) is connected to Sniffer (C) with a null modem (crossed serial cable) on port /dev/ttyUSB1 of Sniffer (C).
Host (B) is connected to Sniffer (C) with a null modem (crossed serial cable) on port /dev/ttyUSB0 of Sniffer (C).
On Sniffer (C), launch the slsnif program with the following parameter:
slsnif -p /dev/ttyUSB0 /dev/ttyUSB1
On the terminal of Sniffer (C), you can see the serial data exchanged between A and B.
On a modern computer without “native” serial ports, you must use Serial/USB adapters.
The important point is to use null modem between A and C and between B and C.
A ——( null modem )——- C ——-( null modem )—— B
A package for Slackware 12.2 is available here: