Download Source Package snooper:
Snooper passes data transparently between two serial (RS232C) devices, capturing and logging the data and occasional comments you want to insert into the logs.
It is useful for debugging or analyzing the communications protocol between two devices that would normally be connected directly to each other, e.g. a digital camera and a personal computer. By sitting "in the middle" (after you connect the two devices to serial ports on your Linux machine) snooper is able to capture data traveling in either direction while also passing it unmodified to the other device.
It is also possible to operate with a single serial device, using your console and keyboard as the second device.