I will state up front: The knowledge I am about to lay down in print was imparted to me by Roger (GW4NOS). I am putting it here so that a) I can find it again; and b) the internet will archive it for future seekers of knowledge.
Like many people I bought a Yaesu FTdx-1200 and it works very well… except when using WSPR software, there seems to be no incantation that permits CAT control to work. Like the Reverend Mother told Paul Atreides:
It turns out, however, that the software can be made to work in a few simple steps.
Step 1: Select Yaesu FTdx-9000 on the WSPR software. Apparently that radio has the same CAT command set as the FTdx-1200.
Step 2: On the radio, menu 039 must match the setting in the WSPR software.
Step 3: On the radio, menu 040 must be 100 ms or greater. Apparently, the default of 10 ms is too speedy for the WSPR software.
Step 4: On the radio, menu 041 must be “DISABLE”. Nothing I have uses CTS/RTS flow control anyway.
Once that was done, everything worked perfectly!