Firstly, I apologise for my initial post - It should have said RA and not TA.
And as I understand you should get RA when another aircraft is within 600 ft even...
That was the minimum margin of separation for TCAS I. But TCAS II attempts to keep a 20-30 second margin between the two aircraft in an RA. But both people HAVE to have TCAS. You will not get an RA is the other guy just has Mode C.
PM