190-00906-00 GTX 33 Installation Manual
Rev. F Page 4-5
4.4 Altitude Functions
Altitude functions with pin assignments are shown for reference since the altitude function is available in
the GTX 33. In the Garmin Integrated Flight Deck system, altitude data is received from the GIA 63 in
RS-232 format.
Parallel gray code altitude inputs are considered active if either the voltage to ground is < 1.9 V or the
resistance to ground is < 375 Ω. These inputs are considered inactive if the voltage to ground is 11-33 Vdc.
The GTX 33 contains internal altitude code line isolation diodes to prevent the unit from
pulling the encoder lines to ground when the transponder is turned off.
If two separate altitude encoders are connected to the GTX 33, one providing parallel
gray code and the other, serial data, the unit selects only one for use at a time, with serial
data input receiving the highest priority.
For altitude encoders that can be connected in both serial data and parallel gray code
format, such as the Garmin GAE 43 (Garmin P/N 013-00066-00), select one or the other
but not both wiring connections.
When connecting two GTX 33 transdponders to a Garmin GNS 480 (CNX80), the unit can
only receive serial data from one transponder at a time. Use a DPDT switch to connect
both serial data and External Standby Select. Refer to Figure D-4.
Among the surveillance items the Mode S transponder will transmit to the ground stations and other
aircraft are altitude reporting in 25-foot increments with the proper encoder. In order to report altitude in
25-foot increments the GTX 33 must receive altitude from suitable altitude reporting devices through serial
input connections. Altitude input to the GTX 33 received from parallel wire gray code encoders is supplied
to the unit in 100-foot increments and thus reported in 100-foot increments.
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