Hi Arrows2100.
Assume that the TARGET aircraft is equipped as below:
MODE A ONLY:
TCAS will not decode any Mode A info as such, but will as you say give you a TA against any Mode A ONLY traffic. (This can also be shown as PROXIMATE TRAFFIC on the aircraft’s TCAS IVSI or EFIS ND). The TCAS Computer interrogates the ATCRBS in a similar way to the ground station, but extracts no IDENT data. Using the range and relative bearing of the target, the TCAS computer calculates the threat and will display as such. TCAS transmits data on 1030Mhz and receives transponder data on 1090Mhz.
MODE C ONLY:
As you probably know, Mode C has barometric altitude GREY CODE data inserted into the pulse train and TCAS will use Mode C absolute altitude data against its own aircraft altitude (i.e. relative altitude) along with relative bearing and range to generate and display target information. This time as well as PROXIMATE TRAFFIC and TA displays, RAs can be generated and displayed in the IVSI or EFIS ND AND PFD). The target aircraft of course will have no indications or protection. (TCAS cannot function without an onboard MODE S transponder, inferring that the target aircraft is not equipped with TCAS).
MODE S:
As MODE C except that avoidance manoeuvres can be co-ordinated by both aircraft, and therefore there are no conflicting RA commands. The data protocol within Mode S is radically different to the old ATC ATCRBS and is far more sophisticated.
I hope that all this blurb is some help to you, and good luck in your NATS Oral.