On Dec 8, 2:48*pm, jcarlyle wrote:
What Darryl said.

You might like to know that the avionics technician who did my VFR
transponder checks went to his computer and got the proper ICAO code
from the FAA. The reason is the "S" in Mode S stands for selective,
and they want to be darn sure ATC is broadcasting to the proper
aircraft.

All seems very reasonable and I would have no intention of using an
incorrect ICAO address. Did your technician also check that the
transmitted registration number, which seems to be a separate data
entry on the TT21, was correct?

Andy