Hogger60.
When you switch on the TCAS & Transponder you DO NOT then get a primary return. You get a sqawk & a height readout (Modes A & C) from the transponder (unless Mode c is switched off - & if it is, you won't get TCAS advisory).
A Primary return is a blip of reflected electro magnetic energy, which contains NO coded information &, therefore, NO a/c identifying info. It is therefore, useless for TCAS purposes.