Sorry another r.a.s. typo. " Sone GA PCAS systems do legacy mode C only. " should be "Some GA TCAD systems do legacy mode C interrogations only." An example system is the L-3 SkyWatch, it's a Mode C interrogator only, no Mode S. And obviously PCAS does not do any interrogations, it just listens to responses from transponders to other interrogations.

BTW Those legacy Mode C only interrogators help mess things up for deduplicatiing 1090ES and FLARM signals from traffic. Since the reply to a legacy Mode C interrogation, even from a Mode S transponder, does not include the aircraft's ICAO address a PowerFLARM can't easily know to suppress the PCAS alert for a target it also has via ADS-B/1090ES or FLARM.