TCAS 1 continues to depend upon an ATC radar sweep activating all aircraft in the area but is able to hear the responses of those other aircraft and tracks and displays their relative position. TCAS 2 adds the ability to transmit an interrogation pulse of its own so forcing other aircraft transponders to report. This is quite important because there is a 7 second interval between surveillance radar sweeps. In addition to display of traffic, it also makes a voice warning of evasive action if required. For an excellent summary document on this check out Transport's internal document Aircraft Certification Staff Instruction 17 on ACAS and TCAS.