TT21/TT22 Transponder Installation Manual
22 August 2017
00560-00
Issue AP
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Trig Avionics Limited
17
signal when it is transmitting, and can be suppressed by other equipment that
asserts the signal. The TT21/TT22 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, and affects both the Mode S reply behaviour and
the ADS-B reporting behaviour. 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 appropriate squat switch circuit this input should be left
unconnected, and the transponder programmed to ignore the input.
Note: On an aircraft without a squat switch, but configured for ADS-
B Out, the transponder can use the GPS data to determine the
Airborne/Ground mode automatically.
5.6.8
External Standby Input
This input, when held low, places the transponder in Standby mode. It should
be used to switch between transponders in an installation with two
transponders. The input is active low, and will be asserted when the voltage to
ground is pulled below approximately 4 Volts.
5.6.9
GPS Position Input
The GPS position input is required to support ADS-B functionality. The GPS
position input is an RS232 input to the transponder. The ADS-B features are
optional – no GPS is required for normal Mode S Elementary Surveillance.
