190-01499-02 GTX 3x5 TSO Installation Manual
Rev. 1 Page 5-6
5.6.2 Altitude Gray Code Input
The parallel Gray code input can be used as a pressure altitude input.
The Gray code inputs are pulled down for active, or open for inactive.
The GTX 3x5 includes internal isolation diodes on the gray code inputs to prevent the unit from pulling the
encoder lines to ground when the transponder is turned off.
5.6.3 Altitude Input Selection Priority
Care must be taken to understand the priority for selecting the pressure altitude inputs. The GTX 3x5
searches the altitude inputs in a defined sequence and stops when it finds a valid pressure altitude input.
5.7 Discrete IO
The discrete inputs and outputs are active low.
The discrete inputs are considered active if either the voltage to ground is ≤ 3.5 Vdc or the resistance to
ground is ≤ 375Ω. The inputs are considered inactive if the voltage to ground is > 6.5 Vdc or the resistance
to ground is > 100kΩ. If the input functions are not needed or not used, they should be left open.
Pin Name Connector Pin I/O
ALT A1 P3251 10 I
ALT A2 P3251 32 I
ALT A4 P3251 53 I
ALT B1 P3251 11 I
ALT B2 P3251 33 I
ALT B4 P3251 54 I
ALT C1 P3251 12 I
ALT C2 P3251 34 I
ALT C4 P3251 55 I
ALT D4 P3251 13 I
GROUND (altitude Gray code ground) P3251 56 O
Pin Name Connector Pin I/O
EXT STANDY P3251 14 I
ALTITUDE ALERT OUT P3251 17 O
TEST MODE P3251 35 I
EXT IDENT (momentary input) P3251 36 I
AUDIO MUTE P3251 37 I
AIR/GROUND P3251 57 I
TIS-A SELECT P3251 58 I
DISC IO1 P3251 15 I/O
DISC IO2 P3251 16 I/O
DISC IO P3252 13 I/O
DISC IO P3252 14 I/O
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