TCAS - one RA and one TA: is it possible?

[fellman]

Firstly we need to clear up some nomenclature.
If you are talking about RAs then I assume that you are talking about TCAS II (TCAS II can issue TAs and RAs, TCAS I only issues TAs).
If you are talking about TCAS II then the aircraft is necessarily equipped with a Mode S transponder - a Mode S transponder on own aircraft is a requirement for TCAS II equipment to function (the other aircraft however can be a Mode C aircraft without TCAS).

So, in an encounter between two TCAS II aircaft (which are therefore Mode S aircraft), it is possible to get an RA in one aircraft and only a TA in the other.
In an encounter between two TCAS II aircraft there is no requirement for one to generate an RA just because the other has generated an RA (although this is what usually happens). IF they both generate RAs then those RAs will be coordinated.