TT22-style transponder

A compact Mode S transponder head in the style of Trig's TC20 for the TT22: a mode knob with OFF, SBY, GND, ON and ALT, a CODE knob with ENT for entering the code digit by digit, and a blue LCD with the code, flight ID, mode and flight level. FN lets you enter a flight ID.

Modeled on: Trig TT22 (TC20 control head)-style Mode S 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

LCD: the code large, a reply mark at its left while replying, IDT while identing
The flight ID (yours from FN, or the simulator's ATC ID) and the mode with FL in ALT
Code entry: the digit being set highlighted; ENT moves on, the last digit sends it; abandoned after 7 s untouched
FN: enter an 8-character flight ID (A-Z, 0-9) on the CODE knob, ENT moves on, a blank ends it
GND position on the mode knob (sends ON, shows GND)
VFR key: the VFR code, pressed again the code before it
Blank when OFF; dark without avionics power; dimmed at night, faceplate under the red flood light

On the iPad

Drag around the mode knob to step OFF / SBY / GND / ON / ALT; tap it for the next position (after ALT, back to OFF)
Drag the CODE knob to start a code entry and change the highlighted digit; tap ENT for the next digit (the last sends the code)
Tap FN to enter a flight ID instead (again to cancel)
Tap IDT to ident; tap VFR for the VFR code

Settings

App Settings (shared by all transponders): VFR code and the automatic ground mode

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`
Indicated altitude (ft)	`INDICATED ALTITUDE (feet)`	`sim/cockpit2/gauges/indicators/altitude_ft_pilot`
On the ground (1/0) (bool)	`SIM ON GROUND (bool)`	`sim/flightmodel/failures/onground_any`
Text: aircraft registration (ATC ID) (text)	`ATC ID (string32)`	`sim/aircraft/view/acf_tailnum`

Sends

When you…	MSFS	X-Plane
Mode knob: OFF (0), SBY (1), GND (3), ON (3), ALT (4)	`SimVar write: TRANSPONDER STATE:1`	`sim/cockpit2/radios/actuators/transponder_mode`
ENT on the last digit, or VFR	`XPNDR_SET`	`sim/cockpit2/radios/actuators/transponder_code`
IDT	`XPNDR_IDENT_ON`	`sim/transponder/transponder_ident`