1. ATC transponder 2 gets 115v ac power from xfr bus 2. The ATC/TCAS control panel gets 115v ac power from both xfr bus 1 and xfr bus 2.
Standby/On Discrete
The ATC/TCAS control panel sends a ground standby discrete to the transponder that is not in use.
Identity Code
The flight crew sets the four digit identity code and it shows on the ATC/TCAS control panel. Both transponders get the identity code from the ATC/TCAS control panel.
Control Data
The ATC/TCAS control panel also sends control data to the transponders. This control data does these functions:
Permits mode C and mode S operation.
Lets the transponder send the special position identification (SPI) pulse or airplane identification code.
Air Data
The air data inertial reference units (ADIRU) send air data to the ATC transponders.
ATC transponder 1 gets air data at AD input 1 from the left ADIRU air data reference (ADR) bus 4. The left ADIRU also supplies air data to the ATC 2 transponder from ADR bus 4. ATC transponder 2 gets air data at AD input 1 from the right ADIRU air data reference (ADR) bus 4. The right ADIRU also supplies air data to the ATC 1 transponder from ADR bus 4.
ATC SYSTEM - MAINTENANCE INTERFACE
PSEU
The proximity switch electronics unit (PSEU) sends air/ground discretes through the ATC/TCAS control panel to the transponders.
These are the purposes of the air/ground discretes:
Prevent ground tests of the ATC system in the air
Prevent mode A and mode C replies on the ground
Defines flight legs in the non-volatile memory.
Transponder Fail Discretes
Each transponder built-in test equipment (BITE) continually monitors the equipment for failures. If the BITE detects a failure, the transponder sends the fail discretes to the ATC/TCAS control panel.
ATC SYSTEM - TCAS INTERFACE AND PROGRAM PINS
Program Pins
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.
Digital Data to TCAS
The active ATC transponder sends this data to the traffic alert and collision avoidance system (TCAS) computer:
The 24-bit mode S airplane identification. TCAS needs the airplane mode S identification for maneuver coordination.
Barometric altitude
Maximum true airspeed
TCAS coordination message data
ATC transponder system failure data.
TCAS to Active Transponder
The TCAS computer sends this data to the active transponder:
General TCAS operational data
Coordination update data.
