Honeywell TPA-100 TCAS Processor with "Suppression Line" fault. The TCAS system test failed.
ATC Transponders, DME Interrogators, and the TCAS processor all use 1030MHz as an operating frequency. A suppression line (co-ax) is connected between all the units. While one unit is in active transmission/reception, it basically tells the others to "shut up" until I'm done. A broken or disconnected suppression line allows for multiple unit operations at the same time. Failures can be possible in any of the associated systems (ATC/TCAS and both DME's).
On this particular aircraft, the TCAS suppression line did not have continuity to the other units.
There were two co-ax "T" fittings and a single co-ax junction used to connect the units.
After the aft E&E wall was pulled down........ the TCAS suppression co-ax was plugged into a "unused" bulkhead connector. After the connector was placed on the "correct" junction..... the TCAS system tested correctly. ( The aircraft had recently completed an ADSB modification.)