Ok, I've done some more research on my device and found out a lot more, and I'm starting to get stuck again.

My basic question is, "how do I use my computer to eavesdrop on some RS485 communication?" but I think I should explain the whole situation here so that those in the know can better anser my question. So here goes:

By looking at how this thing is implemented and compairing that with some research I did on the internet I'm now 95% sure that I'm dealing with a 4 wire (two-twisted pair) implementation of the RS485 protocol as described here.

I'm starting to realize that this may be more complex than I originally thought. Turns out these electronics are the same ones (or at least very similar to ones) used in another, much larger and much more complex network made by the same people (a division of AT&T). In fact this network I'm trying to "eavesdrop" on is actually a sort of node on a much larger RS485 network (however it is separate).

So here is an overview of what I've figured out so far, and then I'll explain where I'm stuck.



(All the devices / boards are plugged into the third-prong ground of an elecrical socket, which might be important.)

I'm not sure exactly how this protocol (what I'm pretty sure is RS485) is implemented or how I can begin to reverse engineer it. I think it works something like this though:

The "bridge" device I've drawn above is the brain of my "sub-network". I believe it acts like the "master" and polls each "slave" device A1-A8, B1-B8, asking (in some way to be determined), "are you busy?" to each slave device at a fairly regular interval. The slave device then responds "yes" or "no".

The master also has the ability to tell a specific slave devices to "start" at which point they become busy for an arbitrary period of time.

I'm not sure how the middle tier fits into that, but I think it's more than just some simple multiplexor because there's a pretty good deal of electronics there. I think what must actually happen is:

1) the master says to the boards: "A board, ask A1 if it's busy"
2) the A board tells the A devices: "A1 are you busy?"
3) A1 responds "No" to A.
4) A tells the master "No".

And the process repeats for other devices.

So I need a computer to be able to understand when the different devices are busy. It would be possible (but ridiculous) to do my own hardware and string new wire to each device. However, if I can get this to work this will probably be implemented on about 40 identical "sub-networks" and it is pretty much a deal-breaker to string new wire to the devices themselves, especially when all the data is passing in and out of the "master" device and should be accessible from there.

So at this point I need to find some info on how to listen to the RS485 communication that's there. I think I'll need to build / buy some hardware to interface with my computer because unlike RS232, RS485 works on a difference in voltage levels in the twisted pair, not a potential difference to ground. (Just for fun I took a DC multimeter to each twisted pair. The potential difference in A/B seemed to be 9 volts from the point of view of the DC multimeter, and it couldn't read C/D (perhaps it was changing too much)).

The whole point of this is I need some computer software to know when each machine is busy by looking at the wires coming out of the "master".

Where should I look to figure this stuff out? Anybody have experience with this stuff? What should I check now?

Many thanks for your infinite wisdom,

John