Having done this way back when, I decided to post what I think was a pretty nifty solution.
(Besides, this could be useful for troubleshooting communications between any two pieces of computer equipment, including an EtherMeter.)
In my case, I had both a master and a slave PLC connected via RS-232 serial (null-modem) cable; and I planned to use my notebook computer’s two serial ports to “snoop” the data transmitted from each of these PLC’s.
I cut open the RS-232 cable and used alligator clips to “snoop” the TxD lines from both PLC’s plus the GND line. I did not break the connection, though.
On my notebook (snooper) COM1, I connected my RxD and GND to the Master PLC’s TxD and GND…. and on my notebook (snooper) COM2, I connected my RxD and GND to the Slave PLC’s TxD and GND.
COM1 would listen to (and log) data from the Master; and COM2 would listen to (and log) data from the Slave.
This eliminated the need to have my “snooper” software receive and re-transmit data. I only needed to listen to, and log data from, each of my notebook’s COM ports.
The reason that this works is because a single RS-232 TxD line is capable of driving two RS-232 RxD lines.
My “snooper” program was written in C++ and compiled with MS VC++ 6.0
Here is a link to the “snooper” source code file…