The TTR-921 Transmitter-Receiver contains circuits for receiving, transmitting, signal processing (including computing), and interfacing with other components of the TCAS-94 system. A frequency source generates 1030 MHz for the transmitter output and receiver local oscillator with a receive frequency of 1090 MHz. The receiver has four individual RF and IF sections, one for each of the four directional segments of the TRE-920 directional antenna. At the receiver detector, the signals are combined to allow received-signal bearing information to be determined. This information is passed to the signal processor. The transmitter is a six-stage, solid-state circuit that develops 1800 watts. The phase of the four output signals shapes and points the beams, or generates an omnidirectional pattern. The TTR-921 requires two antennas; one must be mounted on top of the aircraft fuselage and the other on the bottom.

All TTR-921 units require ARINC 429 control format. Control data is generated in the TCAS control and is supplied through the active Mode S transponder. The TTR-921 is 7.64 inches in height, 7.77 inches in width, 15.26 inches in length, and weighs 18.5 pounds max requiring convection cooling and not forced air.