It was said that the two devices a sending information to each other. First I would sniff the data with two serial ports on my PC to see what is being sent back and forth. Unfortuntely I was not born with super powers that can see the electrons flowing in wires!

Once you see what is happening, the solution should be more obvious. WinTech has a tricky comms sniffer program that has a demo that I use in cases like this. We all like to know that our code will work the way we intended it but many times reality doesn't see it the same way!