Evvy

ACAS is a generic term and, so far, TCAS is the only system within the category.
TCAS needs the other aircraft to be transmitting at least mode C in order to issue an RA, although TAs should be available on receipt of mode A transmissions. In order to facilitate fully programmed RAs (where both aircraft respond; typically one CLIMB and other DESCEND in a head-on situation) both aircraft must have serviceable mode S transponders. If one has u/s mode S or merely mode C only the serviceable mode S equipped a/c will receive an RA. This may well rapidly escalate to an INCREASE CLIMB/DESCENT because the other a/c has 'failed' to respond.
Someone else will have to clarify with regard to military a/c - I believe they're mode C.

rts

Rodthesod

My earlier post clearly states my understanding of what triggers the full cockpit TCAS aural and visual alerts.

To clarify therefore my question, the Greek aircraft took avoiding action, was this from an "s to s" contact (do all military operate Mode S?) or from a "c to s" (pilot initiated descent?)

Evvy

Sorry to split hairs, but TCAS does not see mode A transponders at all. It sees Mode S or Mode C, either with altitude data (in which case an RA can be generated) or without altitude data (in which case it's TA only).

You have to have mode C (or S) to send altitude data, but you don't have to have altitude data to reply to a mode C interrogation, you just send an empty pulse frame which is enough for tcas to do a speed of light / range calculation. TCAS doesn't interrogate mode A - the 4096 code is useless to it and just wastes bandwidth.

CPB