Overthewing, all of the TCAS units I've played with have been integrated with the transponder. Turn the transponder off and you don't have a TCAS, at all, not for receiving or transmitting. Think of TCAS as a type of transponder rather than a separate system.
Thanks for this. Not sure this is relevant to the current thread, but does that mean that your working TCAS becomes inop if you switch the transponder to Standby?