BXP6401-style transponder

A 57 mm Mode S transponder in the style of the Becker BXP6401: an OFF / SBY / ON / ALT rotary switch, the code edited digit by digit on the encoder, and a blue-white LCD with the code, flight level or flight ID, and R / ID / mode annunciators. SEL pages include the VFR code you can store.

Modeled on: Becker BXP6401-style 57 mm 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; FL and the flight level in ALT, otherwise the flight ID (the simulator's ATC ID)
R while replying, ID while identing, and the mode (SBY, ON, ALT, GND) down the left
Code edited a digit at a time (0-7), sent after the last digit; abandoned after 10 s untouched
SEL pages: VFR= (editable, STO stores it as the app's VFR code), AI (flight ID), FN, AA, MA, AT, CFG brightness
VFR key: the VFR code, pressed again the code before it
SEL held shows IBIT
Display dark without avionics power; dimmed at night, faceplate under the red flood light

On the iPad

Drag around the rotary switch to step OFF / SBY / ON / ALT; tap it for the next position (after ALT, back to OFF)
Drag the encoder to change the highlighted code digit; push it for the next digit, past the last it sends the code
Tap IDT to ident; tap VFR for the VFR code
Tap SEL to open the pages (encoder turns through them or edits VFR / brightness); tap STO on VFR= to store it

Settings

VFR code: editable on the unit (SEL > VFR=, STO), saved as the app-wide VFR code
App Settings: 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
Rotary switch: OFF (0), SBY (1), ON (3), ALT (4)	`SimVar write: TRANSPONDER STATE:1`	`sim/cockpit2/radios/actuators/transponder_mode`
Encoder pushed past the last digit, or VFR	`XPNDR_SET`	`sim/cockpit2/radios/actuators/transponder_code`
IDT	`XPNDR_IDENT_ON`	`sim/transponder/transponder_ident`