Hi,
There are a ton of serial port monitor/sniffer software around but they are usually aimed at sniffing communication going into and out of the PC on which the port monitor is running. If you're trying to sniff the communications between two devices where the PC running the sniffer software isn't one of them you'd need two serial ports and the software would need to keep the data in sync. I don't know any software that does this but I'm sure it exists.

Now, if you don't need to see the data in real time I'd suggest you get a LOGIC from Saleae. If I understand the problem correctly you could possible set an output when the serial timeout trips, trig the logic analyzer off of that signal and have it setup to have enough pre-trigger data stored to be able to see what was being sent back and forth between the devices. Another option is of course to just sample the data, store it and hope that it errors out before the memory runs out - which, depending on sample rate etc can actually be quite some time.

Unfortunately the LOGIC isn't able to trig off of specific serial data etc, (since it isn't analyzing it in real time) but it's still a VERY usable device IMO.

/Henrik.