User Manual

system components 523-0 809018
2.7. TDR-94D 34-54-00.
The sections that follow describe the external theory of operation, internal theory of operation, and general data of the Mode S
transponder in a traffic surveillance system.
2.7.1. M
ode S Transponder, External Theory of Operation.
NOTE
The term CONTROLLER refers to any transponder control unit, such as an Radio Tuning Unit (RTU), a Transpon-
der Control (CTL), or a Control Display Unit (CDU). Buses and discretes are followed in parentheses by the spe-
cific bus name shown in the example illustration, Figure 2-7.
Refer to Figure 2-7. This s heet shows a typical t ransponder system, and the external signals to and from the Transponder (TDR).
Operating power is derived from the +28 V dc avionics triple-fed bus supply.
2.7.1.1. The TDR is controlled by an ARINC standard transponder controller such as an RTU or a CDU. A controller provides the
transponder state (STANDBY, ALTITUDE REPORTING OFF, ALT ITU DE REPORTING ON), the flight ID, the squawk code, the
selected altitude, and TCAS control data.
2.7.1.2. Either tw o or three ARINC 429 low-speed control busses may b e routed to the TD R containing co ntrol a nd altitude data.
The on-side data comes to Control Port B. The cross side data comes to Control Port A. An optional back up third controller comes
to Control Port C. The microprocessor reads the discrete (RTU PORT A/B SELECT, P2-57) and selects the active port. Normally,
port B is selected and the TDR is tuned by the on-side controller (RTU). When the reversion switch (RTU1 INHB) is set to inhibit, a
ground is applied to pin P2-57, which selects port A of the right side controller (RTU) tune data. When the reversion swi tch (RTU2
INHB) is also set to inhibit, a ground (BURST TUNE ENABLE) is applied to pin P2-59, which selects port C (IAPS) burst tune
data.
2.7.1.3. The TCAS/transponder cross talk busses (TSS-TX and TDR-XT busses) are used for coordination between the TCAS
function of the TSS and the TDR.
2.7.1.4. The Mode S address is set by rear connector strapping (P1-33 through P1-56) and is unique for each aircraft.
2.7.1.5. The main transponder output bus (TDR-1) outputs maintenance data and echoes selected control data.
2.7.1.6. The TDR-94D operates with two antennas to provide traffic air-to-air capability. The 1030 Hz interrogation input is
received on either or both L-band antennas, low-pass filtered, and applied through diversity and transmit/receive switches to the
front-end receiver. The diversity switch is not used in receive mode. The transmit/receive s witch connects the antennas to the re-
ceiver (not the transmitter) in receive mode.
2.7.1.7. The TDR responds to Air Traffic Control Radar Beacon System (ATCRBS) and Mode S All-Call interrogations.
2.7.1.8. The TDR also responds to selective Mode S interrogations. These interrogations use the transponder Mode S address.
This mode allows th e air traffic controller to identify each aircraft by tail number. It is also u s ed by the traffic surveillance s ystem
for air-to-air communication with a co operating aircraft.
2.7.1.9. A suppression pulse (P1-29) is generated while the TDR is transmitting a response to a n interrogation. This pulse is
intended to inhibit the receivers in other L-band radios, such as the TSS and the DME. These other radios also generate a suppression
pulse while they are transm itting. While this is active the TD R will inhibit its replie s.
2-21/(2-22 Blank)