I was trying to demonstrate Com 1 transparent use by configuring the Cntrl USB to be Com 1, then attaching an ICOM CT-17 level converter on the Radio COM 1 port using Serial Settings Mode 2 for both ends, while keeping the Radio configuration for ICOM 706 and having the radio remote working at the same time.
Try as I might, despite matching baud rates, etc, I was unable to get it to work.
I switched the USB to be Com 2, used the Com2 port on the radio end to the CT-17, and it worked fine through HRD and the remote simultaneously.
I'll leave it be since it works but the manual suggests Com 1 should work fine but it didn't. I did have to use a gender changer on the radio end for Com 1 due to the DB9 cables I had but they are straight cables, no crossover to my knowledge. So I guess my advice is just use Com 2.