Thanks Graybeard
The question arose in another forum as to whether there were transponders around that wouldn't reply to a Mode C interrogation (in that case from TCAS) with at least framing pulses. I've managed to establish that:
* Since 1975 all operating transponders have to conform to TSO-C74b or TSO-C74c
* TSO-C74c definitely requires reply to a Mode C interrogation with framing pulses even in the absence of altitude information
I'd like to be able to say that means that all legal transponders reply to a Mode C interrogation with framing pulses (and thus show on TCAS). However, TSO-C74b is now "historical" and the FARs are phrased as follows:
"For operations not conducted under part 121, 127 or 135 of this chapter, ATC transponder equipment installed must meet the performance and environmental requirements of any class of TSO-C74b (Mode A) or any class of TSO-C74c (Mode A with altitude reporting capability) as appropriate..."
That left a slight doubt in my mind as regards the response of a TSO-C74b transponder to a Mode C interrogation. My questions about TSO-C74 and TSO-C74 were merely curiosity as to the history of development.