>>>I sort of have this working now. Two problems...
>>>
>>>1. When I change the senders RTS line, the receiver (VB activex DLL) oncomm fires three times.
>>>
>>
>>Sure - you're causing multiple events since you're 'pinging' the handshake line - OnComm sees two transits of the CTS line (RTS crosses CTS), which issues two comEvCTS, and probably a Send buffer not full (comEvSend) report.
>>
>
>I'm not sure why it would see two transitions. Just as a test I set it from high to low and with that I get three events. I have logged all
>the events by capturing and appending the event number to a string. I get three 3s in a row. I have not sent any data in this test.
>

3 is a comEvCTS, so you transit from default CTS state to high to low; I'd check the value of the CTSHolding property on every 3.