I seem to remember that was dealt with by the BEA in Interim Report #2. From memory, the TCAS has an inbuilt function which checks that the static port pressure is valid and that the calculations that result fall within predetermined values. Should the TCAS detect erroneous values, it fails.
Altitude and its valid is transmitted on a data bus from the Air Data part of the ADIRU to the transponder. The transponder transmits own altitude on another data bus to the TCAS, and over the air to ATC and other aircraft.
Airspeed is not part of the TCAS calculation. TCAS uses only altitude difference, distance, and rate of closure.
If altitude data is flagged as failed, it does not cause a failure of the transponder or the TCAS. The transponder reverts to Mode A, and the TCAS annunciates OFF.
A TCAS Fail message means the TCAS has failed; it does not mean that it is turned off by an input fail or pilot selection.