User's Manual

TT21 Transponder Installation Manual 19 January 2009
00560-00 Issue AA
______________________
5.6 TT21 Transponder Interface Details
5.6.1 Power Input
The power supply can be 11-33 Volts DC; no voltage adjustment is required.
Use a 3 Amp circuit breaker for power supply protection to the TT21.
5.6.2 TMAP Bus
TMAP is a Trig proprietary bus based on RS485 signalling. It provides a bi-directional interface
between the transponder and the control head. Each TMAP interface comprises a balanced pair of
signals, called A and B, both of which must be connected for communication to work.
There are two sets of TMAP pins on the TT21, TMAP1 A and B, and TMAP2 A and B. This is to
support future installation wiring options. The two sets are identical, and either pair (TMAP1 or
TMAP2) may be used to connect to the control head.
5.6.3 Power On
The TT21 power supply can be directly controlled by this discrete input. The input is active low, so
that the TT21 will power up whenever the input is held low. This signal is intended to connect to the
TC20 Remote ON output.
5.6.4 Controller Power
The TC20 controller does not connect to aircraft power – it receives power from the TT21 using this
output. The TT21 provides 6.5 Volts to the TC20. This output includes short circuit protection, and
no fuse is required between the TT21 and the TC20.
5.6.5 Mutual Suppression
Mutual suppression allows two or more transmitters on adjacent frequencies to inhibit the other
transmitters when one is active to limit the interference effects. It is commonly used between
transponders and DME systems, and between transponders and collision avoidance systems.
The Suppress I/O on pin 18 is an ARINC compatible suppression bus interface, which acts as both an
input and an output. The TT21 will assert this signal when it is transmitting, and can be suppressed by
other equipment that asserts the signal. The TT21 will drive approximately 24 Volts on the output
(independently of supply voltage), and will treat the input as active whenever the bus has greater than
10 Volts.
5.6.6 Ident Switch Input
The Ident switch input allows the IDENT function to be selected using a remote switch. The input is
active low, and will be asserted when the voltage to ground is pulled below approximately 4 Volts.
5.6.7 Squat Switch Input
The Squat switch input allows the transponder to automatically switch between Airborne and Ground
modes. The input will be asserted when the voltage to ground is pulled below approximately 4 Volts.
The logical sense of the input can be programmed to be either active low or active high.
On an aircraft with no squat switch this input should be left unconnected, and the transponder
programmed to ignore the input.
Trig Avionics Limited Page 9