Yes, sensing the 12V from the TX/RX line is also an option.
But I'm still a bit reluctant as I'm not completely convinced yet that we
anyway need a two-resistor solution.
The elegantly simple one-resistor method has worked fine in all cases so
far (at least that I know of). So far only one builder (ja9mat Hidehiko)
reported that in his case it didn't work reliably.
As he mentioned that his radio was still in an experimental setup
situation (perhaps with long or loose wires, poor ground connections,
etc.), there is still a possibility that his problem will disappear once
everything is properly built together and wired correctly.
So before overreacting and advising everyone to add extra resistors I'd
like to hear if there are any other builders having similar issues, and if
Hidehiko-san's problem disappeared after he has properly wired everything.
73 Allard PE1NWL
Allard PE1NWL