GTX 327-style transponder

The 172's transponder in the style of a GTX 327: mode keys around ALT, the code typed on the 0-7 keys, and a smoked-glass display with yellow segments showing the mode, the code, the reply indicator and the flight level. It's a 2 x 1 stack unit and a default unit of a new free rack.

Modeled on: Garmin GTX 327-style transponder (as in X-Plane 12's Cessna 172 SP GNS panel)
Category: Radios and panels
Size: 2 × 1 cells
Version: 1.0 · 2026-10-19
Plan: Pro
Simulators: MSFS 2020 · MSFS 2024 · X-Plane 11 · X-Plane 12

Features

Yellow seven-segment code with faint unlit segments; while typing, the digits typed then dashes
Mode shown at the left (OFF, STBY, ON, ALT, or GND on the ground with the automatic ground mode)
R flashes while replying: as radar sweeps pass (every 4.8 s and 12 s), and while identing; not on the ground or in standby
FL and the pressure altitude in hundreds of feet in ON and ALT
TEST held lights every segment (8888, FL 888, R)
IDENT shown as replying for 18 s even if the simulator's ident flag comes late
Code shown immediately when sent, until the simulator confirms it
Display goes dark when the avionics power is off; dimmed (not tinted) at night, faceplate under the red flood light

On the iPad

Tap IDENT to ident
Tap ON, ALT, STBY or OFF to set the mode
Tap four of the 0-7 keys to enter a code: it's sent on the fourth digit
Tap CLR to erase the last digit typed
Hold TEST to light all the segments

Settings

App Settings (shared by all transponders): VFR code (1200 or 7000) and the automatic ground mode (GND on the ground) on/off

Keys and controllers

Map any of these to a key, a game controller's button or axis, or a MIDI knob (how):

Transponder IDENT
Transponder STBY
Transponder ALT

Simulator data

What the Hub reads from the simulator for this instrument, and what the instrument sends back. Names are the simulators' own: MSFS SimVars and events, X-Plane datarefs and commands.

Reads

Value	MSFS	X-Plane
Transponder mode: 0 off, 1 standby, 2 test, 3 on, 4 altitude (enum)	`TRANSPONDER STATE:1 (number)`	`sim/cockpit2/radios/actuators/transponder_mode`
Transponder code as its four digits (e.g. 1200) (code)	`TRANSPONDER CODE:1 (number)`	`sim/cockpit2/radios/actuators/transponder_code`
Transponder identing (1/0) (bool)	`TRANSPONDER IDENT:1 (bool)`	`sim/cockpit2/radios/indicators/transponder_id`
Pressure altitude (ft)	`PRESSURE ALTITUDE (feet)`	`sim/flightmodel2/position/pressure_altitude`
On the ground (1/0) (bool)	`SIM ON GROUND (bool)`	`sim/flightmodel/failures/onground_any`

Sends

When you…	MSFS	X-Plane
IDENT	`XPNDR_IDENT_ON`	`sim/transponder/transponder_ident`
OFF (0), STBY (1), ON (3), ALT (4)	`SimVar write: TRANSPONDER STATE:1`	`sim/cockpit2/radios/actuators/transponder_mode`
Fourth digit typed on the 0-7 keys	`XPNDR_SET`	`sim/cockpit2/radios/actuators/transponder_code`