Each ATC transponder has these program pins:
· Airplane mode S address
· Source destination identifier (SDI)
· Maximum true airspeed.
There are 24 airplane address program pins on each transponder. A program switch module connects to the program pins. The program switch module supplies the open and ground connections necessary to make the 24-bit unique airplane identification.
The SDI defines the transponder as the ATC 1 or ATC 2 transponder.
The maximum true airspeed pins define the maximum true airspeed for the airplane. The transponder formats and sends the maximum true airspeed to the TCAS computer.