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 ATC/TCAS - Control Unit Figure 010 
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 According to the 1/2 selection, the processor 1 or 2 controls the functions of the unit. Each processor can compensate for the other in case of failure. The selected processor reads the switch selections and :
 -drives the LCD display
 -formats the ARINC control word. In case of transponder failure, two independent circuits control :
 -
the failure indicator light

 -
the failure mode of the processor.

 C. Antenna (Ref. Fig. 011) The L-band antenna is a short stub, all aluminum blade type which is completely sealed to prevent failure from moisture incursion. It is vertically polarized, has an impedance of 50 ohms, a Voltage Standing Wave Ratio (VSWR) of 1,5 : 1 and operates in the 960 MHz to 1220 MHz frequency band. Lightning protection is provided to prevent damage to the antenna and the transponder. Four antennas must be provided on the aircraft. The desire for improved upper hemisphere coverage usually leads to a choice of upper and lower mounting locations for the transponder antennas on the aircraft.
 7. Operation_________
 A. Control
 (Ref. Fig. 004)

 (1)
 In normal operation, you select the ATC 1(2) through the 1/2 switch on the control unit in either position 1 or 2. A STBY/AUTO/ON selector switch enables the functions given below:

 -
in STBY position: the two transponders are electrically supplied but do not operate

 -
in AUTO position: the selected transponder operates in flight but operates only in mode S on ground

 -
in ON position: the selected transponder operates when the aircraft is in flight or on ground.

 (2)
 Upon energization of the aircraft or modification of system selection, the last ATC code selected is displayed before the new code selection. This code comes into view on the display window of the control unit. When you set the ALT RPTG/ON/OFF switch to ON, the altitude information transmission of the selected transponder is enabled.

 (3)
 When you push the IDENT pushbutton switch (ON configuration), this generates an additional identification pulse in the reply signal.

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 ATC - Antenna Figure 011 
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 (4)
 With failure of the ATC/TCAS control unit, the transponder continues to transmit the last displayed code. No new code selection is possible.

 (5)
 In addition, a suppressor coaxial connects the ATC tranponders to the Distance Measuring Equipment (DME) interrogators. This prevents transmission from one system while the other is in reception mode. This is necessary because the ATC and the DME systems operate on the same frequency range.

 B. Reconfiguration Switching (Ref. Fig. 004) In normal configuration, each ATC receives the altitude information from its corresponding Air Data/Inertial Reference Unit (ATC1 from ADIRU1, ATC2 from ADIRU2). With failure of the ADIRU corresponding to the serviceable transponder, the pilot can select the altitude information from the ADIRU 3. This selection is through the AIR DATA selector switch installed on panel 8VU on the center pedestal.
 8. Test
____

R **ON A/C 001-049, 101-105, 151-199,
 A. Maintenance Test (Ref. Fig. 012) It is possible to do the maintenance tests with the centralized fault-display system (CFDS). The primary components of the CFDS are :
 -
the Multipurpose Control and Display Unit (MCDU)

 -
the Centralized Fault Display Interface Unit (CFDIU)

 -
the Built-In Test Equipment (BITE) internal to the ATC transponder.
 It is possible to perform these tests on the ground only.
 To get the ATC 1(2) page on the MCDU:

 -select of the SYSTEM REPORT/TEST item on the CFDS menu

 -then, push the line key adjacent to the ATC 1(2) indication on the NAV

 menu. The test sequence starts when you push the line key adjacent to the TEST indication on the ATC 1(2) page. After the INTERFACE TEST selection, the message : PLEASE CHECK IF THE CONTROL PANEL SWITCH IS ON SYSTEM (1 OR 2) AND PRESS TEST KEY is displayed on the MCDU.
 (1)
 Not Applicable

 (2)
 Not Applicable

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 ATC - Maintenance Test Procedure
 Figure 012

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