gpn01
Yes, the £10k and up "TCAS" systems are active interrogators. Your aircraft (if thus fitted) emits an omnidirectional burst every so often, triggering nearby transponders. Only Mode C or S transponders return a useful return though.
Actually "TCAS" is a wrong term as it implies the pilot gets an RA (resolution advisory) which is a voice telling him to climb or descend. The GA kit is not certified to do that. I think TCAD is a more correct name; you just get a screen showing other traffic, and warnings if somebody is getting dodgy. But the pilot decides what to do about it.
The cheapest Zaon box is IMHO crap because you get no azimuth info. If you fell asleep it might wake you up... The next one up (c. £1000) gives you rough azimuth info and should work well. I have flown with a couple of the latter; one kept picking up the aircraft itself, plus traffic, and the other one worked apparently well.
The problem is that it isn't easy to install it neatly - it really needs fixed wiring and a decent mounting surface, and the fixed wiring is a "grey area" in certification terms. I installed a Garmin 496 for its TAWS function (to get a "GPWS" functionality) and the resulting very neat wiring took the avionics man all day to do, and that was after I prewired a lot of cables with special connectors. In fact the 496 could have been mounted completely out of sight (with its on/off switch hot-wired or whatever) but I put it on the yoke and it forms a useful emergency backup GPS.
The whole issue of installing "portable" kit neatly, using what is inevitably fixed wiring and connectors, is a grey area (because the kit you are installing is not approved for a fixed installation, not coming with Approved Data) and needs a very friendly avionics shop and some technical knowledge, for fitting special inline connectors in the cables, so the "portable" kit can be removed for the Annual and then put back in. A lot of people have done it, but most wouldn't, and it needs to be done in a way which would enable you to get the Annual done at even the most anally retentive shop. Easier on the N-reg.