The TPU-66A TCAS Processor is used in the CAS-66A TCAS I system. The TPU-66A contains circuitry for functions of the TCAS I system and interfacing with configuration modules, antennas, graphic processors, controls and displays. The CAS-66A works by interrogating the ATC transponders of up to 30 surrounding aircraft and displaying their positions. The CAS-66A then processes this information and displays the positions of surrounding aircraft to the pilot. This information is provided through a range of display and control options, from a dedicated flat-panel color Traffic Advisory/Vertical Speed Indicator (TA/VSI) with bezel-mounted controls, to a weather radar, multifunction display (MFD), or Electronic Flight Instrument System (EFIS) interface with a separate control panel.

The TPU-66A is designed for installations with top-mounted directional antenna ANT-67A, which is always required, plus an optional bottom-mounted directional antenna ANT-67A. The TPU-66A is 4.98 inches wide, 7.70 inches high, 12.56 inches in length and weighs 17.2 pounds. The primary power input for the TPU-66A is 28 VDC. The TPU-66A's operating temperature is -55°C to +70°C and is convection cooled. The TPU-66A has a warm up time of 3 minutes for frequency stabilization.