Oh well, can't be spot on all of the time
Yes, Mode C altitude is required for RA, TA otherwise. Thank you for the correction.
This link is quite useful from a google search, guide to TCAS by ARINC -http://www.arinc.com/downloads/tcas/tcas.pdf
What I should have said is that those aircraft incapable of providing Mode C data (regardless of whether an altitude encoder fitted or not) cannot be detected by TCAS. The transponder must be capable of providing Mode C replies to be seen by TCAS (which will rely on detecting the F1-F2 framing pulses only if no altitude info) so those older Mode A units which were never designed to provide Mode C replies cannot be seen by TCAS. I shouldn't think that there are that many of these still around, but you never know.
Anyway, the Mode S transponder used by TCAS is only requesting Mode C information, not Mode A, to track non-Mode S equipped aircraft as I said.
