During my career I didn't have much opportunity for hardware programming other than TAPI. I've done serial port programming before but client port settings were clearly defined by the server. With the FT-991A the client port settings are not defined beyond setting the baud rate.

In February 2022 a reset was done to my FT-991A. Connectivity with the radio was lost until a kind soul suggested disabling extended menu selection #33 aka CAT-RTS (Ready To Send). With that one change after the reset connectivity was restored. In late March the extended menu backup/restore utility was released. One user couldn't get the utility to work until CAT-RTS was disabled on his radio.

At that point the little light came on. The .NET serial port type's RtsEnabled property is set to false by default. Some experimentation resulted in the truth table below.

By enabling the serial port object's RTS property connectivity will be had regardless of the rig's CAT-RTS setting. Lesson learned.