TRT800-style transponder

A compact transponder for a 57 mm round hole in the look of a f.u.n.k.e. TRT800: the mode and the code on a gray LCD, FL and the pressure altitude in ALT, the reply indicator. Set the code digit by digit with the knob.

Modeled on: f.u.n.k.e. (Funkwerk) TRT800-style transponder
Category: Radios and panels
Size: 1 cell
Version: 1.0 · 2026-10-19
Plan: Pro
Simulators: MSFS 2020 · MSFS 2024 · X-Plane 11 · X-Plane 12

Features

Mode (SBY, ON, ALT, GND, TST) and the four-digit code, the digit being edited underlined
FL and the pressure altitude in feet in ALT mode
R while replying; IDENT while identing; emergency codes named (HIJACK, NO RADIO, EMERGENCY)
MODE key cycles SBY, ON, ALT; I/O turns it off or back to standby
VFR key sets the VFR code from Settings (again: the previous code)
Automatic GND mode on the ground (per Settings)
LCD blank when off; dimmed at night; dark without avionics power; the faceplate takes the red flood light and, when its dimmer is up, the glareshield's warm light (Settings > Dimmers > Glareshield, off by default)

On the iPad

Drag the knob to change the underlined digit (0-7, wrapping); tap the knob or the right-arrow key for the next digit
Tap the up/down key for the previous digit
Tap MODE, I/O, VFR, ID

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`
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
Each knob step on a digit (the full new code), or VFR key	`XPNDR_SET`	`sim/cockpit2/radios/actuators/transponder_code`
MODE key (SBY -> ON -> ALT) and I/O key (off / standby)	`SimVar write: TRANSPONDER STATE:1`	`sim/cockpit2/radios/actuators/transponder_mode`
ID key	`XPNDR_IDENT_ON`	`sim/transponder/transponder_ident`