Mode C: I can't remember having seen a mode C OFF posibility but I figure the text is valid for a L / R only system as well.
FCC/AIR DATA COMPUTER INTERFACE: Shame on me - if I had bothered I would have found that in my own books as well.
Now for the regulations:
JAR-OPS 1.872 Equipment for operation in defined airspace with Reduced Vertical Separation Minima (RVSM) (See JAA Administrative & Guidance Material, Section 1, Part 3, Leaflet 6)
(a) An operator shall ensure that aeroplanes operated in RVSM airspace are equipped with:
(1) Two independent altitude measurement systems;
(2) An altitude alerting system;
(3) An automatic altitude control system;
and
(4) A secondary surveillance radar (SSR) transponder with altitude reporting system that can be connected to the altitude measurement system in use for altitude keeping.
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Re (4) if you use the RH transponder you must also use the R-CMD.
Thanks!