User's Manual

TT21/TT22 Transponder Installation Manual 13 July 2009
00560-00 Issue AF
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Page 16 Trig Avionics Limited
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.
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.
The TT21/TT22 GPS input can recognise the industry standard “Aviation”
format, the NMEA 0183 format, or the format used by certain Freeflight and
NexNav GPS sensors; the interface speed can be selected between 4800, 9600
and 19200 bps.
There is also an RS232 output on the connector for future expansion. This pin
is unused and should be left unconnected.