GTX 345-style transponder

A transponder laid out as a GTX 345: the round mode cluster around IDNT, the code keys, and a display with the mode, the code and a function page. Code, mode and IDENT go to the simulator.

Modeled on: Garmin GTX 345-style transponder
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

Mode cluster: ON, ALT, VFR, OFF, SBY around a round IDNT key
Code entry on the 0-7 keys: digits blink in cyan until the fourth is typed, then it's sent; CLR deletes one; entry times out after 6 s
CRSR cursor editing: pick a digit, change it with 8 / 9, ENT sends the code
FUNC pages: PRESSURE ALT, FLIGHT TIME, ALT MONITOR, COUNT UP, COUNT DOWN, DENSITY ALT, OAT
Altitude monitor: ENT captures the altitude; ALTITUDE ALERT and a caution tone 200 ft off it
Count-up and countdown timers (ENT start/stop, CLR reset, 8/9 set the countdown minutes; tone at zero)
R reply indicator flashing as simulated radars sweep past; IDNT shown for 18 s after IDENT
GND automatic ground mode; emergency codes flagged on red (HIJACK, NO RADIO, EMERGENCY); 7500 asks for confirmation
TEST lights every segment (8888)
VFR key sets the VFR code; pressed again brings back the previous code

On the iPad

Tap a key of the round cluster for ON / ALT / VFR / OFF / SBY; tap the middle for IDNT
Tap 0-7 to type a code (four digits send it)
Tap FUNC to page through the function pages; ENT / CLR act on the page
Tap CRSR to edit one digit at a time, 8 / 9 to change it, ENT to send

Settings

App Settings > Transponder: 'Transponder VFR code' 1200 (US) or 7000 (Europe), shared by every transponder's VFR key
App Settings > Transponder: 'Transponder ground mode' on/off (on: in ON/ALT on the ground it shows GND and doesn't reply)

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 code as its four digits (e.g. 1200) (code)	`TRANSPONDER CODE:1 (number)`	`sim/cockpit2/radios/actuators/transponder_code`
Transponder mode: 0 off, 1 standby, 2 test, 3 on, 4 altitude (enum)	`TRANSPONDER STATE:1 (number)`	`sim/cockpit2/radios/actuators/transponder_mode`
Transponder identing (1/0) (bool)	`TRANSPONDER IDENT:1 (bool)`	`sim/cockpit2/radios/indicators/transponder_id`
On the ground (1/0) (bool)	`SIM ON GROUND (bool)`	`sim/flightmodel/failures/onground_any`
Pressure altitude (ft)	`PRESSURE ALTITUDE (feet)`	`sim/flightmodel2/position/pressure_altitude`
Indicated altitude (ft)	`INDICATED ALTITUDE (feet)`	`sim/cockpit2/gauges/indicators/altitude_ft_pilot`
Outside air temperature (degC)	`AMBIENT TEMPERATURE (celsius)`	`sim/cockpit2/temperature/outside_air_temp_degc`

Sends

When you…	MSFS	X-Plane
Fourth digit typed, ENT after CRSR editing, or VFR key	`XPNDR_SET`	`sim/cockpit2/radios/actuators/transponder_code`
ON (3), ALT (4), OFF (0), SBY (1) keys	`SimVar write: TRANSPONDER STATE:1`	`sim/cockpit2/radios/actuators/transponder_mode`
IDNT key	`XPNDR_IDENT_ON`	`sim/transponder/transponder_ident`