[looking to tap this excellent resource of intellegence ]

I've run across this situation several times while playing around with stuff (and once at work) and never figured out how to address it. I need to reverse engineer some non-standard hardware communications protocol for some reason. The biggest problem is that the topic is so broad and I know so little that I have thus far failed to find good source of info on the web.

Generally, I'm in one of the following situtations. Something simple isn't working right and I'd be willing to consider actually looking at the bits. (rare) I'm curious about the way something works. (somewhat common) I'd like to extract some information from a data stream to use in a different way (on my computer generally.) (common)

What kind of hardware is neccessary to observe and log the actual bits flying around onto my computer? In some cases I've known the protocol to be serial, but I don't know how to get that info into my computer when two hardware devices are communicating without disturbing the data transfer. (I've used serial port loggers before, but I'm talking HW-HW stuff here.) (Other times I've dealt with things that I think used some custom communcation protocol, but its always (with the stuff I've wanted to play with) low speed and not too much data to overwhelm.)

In the case that currently interests me we have two devices that are capable of sending at least the following information to each other.

Device A: Are you availible?
Device B: No.
Device A: Are you availible?
Device B: No.
Device A: Are you availible?
Device B: Yes.
Device A: Process the input "4" (or something really basic.)

There's probably some other stuff going on there in terms of handshaking and other talking but thats all thats there "for sure".

So my goal is to be able to monitor whats going on in terms of their communication. Eventually in real time in some custom software, but for now I'd just like to figure out how they work and then later I can figure out how I might be able to interface them with some software. (By that I just mean observe them. I'll never have to send data or otherwise disturb them.) For the purposes of this excercise, I'm not allowed to modify either device.

So obviously I have a lot to learn. I've been trying but I haven't found any helpful resource on the subject. If anyone could give me some advice that could point me in the right direction it would be much appreciated.

I love this board.

Thanks in advance,

John