TCAS was first made mandatory in the USA.
Here in Europe the TCAS was made mandatory from the 31 march 2001 (but some airliners has got a new deadline due to problems during installation)
The first certified system was called TCAS, Traffic Collision Advisory System, and whats in use today is TCAS II.
Here in Europe the TCAS has also got Called ACAS (Airborne Collision Advisory system) this is due to that the JAA has gone for a software that by the present time is version 7. System are fully compatible with each other. The afterinstallation systems with IVSI are often called ACAS
There are two different layouts in the cockpit.
The factory installed one:
TCAS info are displayed on the EHSI or The ND display, Resolution Advisory info on the PFD or EADI.
This is the best way to display the info.
Then there are the afterinstalled one.
If the airline does not go for a full retrofit to the factory installation (expensive). This would require, new Symbol generators, complete new wiring up to the displays and so on.
If the wiring wasnt there from Boeing, Fokker or MD, then there is another (cheaper) option.
You install two ATC mode S transponders (If not allready onboard the A/C) two Tcas antennas, one on top of the fuselage and one at the bottom. Change the ATC control panel to a ATC/TCAS controll panel. a TCAS/ACAS computer add some wiring in the e&e and up to the flight deck. And instead for the VSI indicator you install a IVSI (Integrated Vertical Speed Indicator) that are able to display both VSI and TCAS info and Resolution Advisory.
A small display and it will give you the info you will need.
The audio alert that the TCAS will give are heard in the cockpit roof loudspeakers, not able to mute or adjust volume on. This is the same for both factory installed or afterinstalled TCAS/ACAS.
Hope someone understand my explanation.