A radio wired to your navigation instrument stops being a separate box you reach across the cockpit for. The instrument can read what the radio is tuned to, set the standby frequency, swap the two over, and — the part everyone actually wants — drop the frequency of the airfield you have just selected on the navigation page straight into standby, without you reading digits off a plate at 500 feet.
This article covers what that integration does and does not do, how it is wired and configured on the Era, LX 10K and Colibri X, and how it differs on NAVIA, where a radio is one of several declared devices rather than a special case. It also deals honestly with transponders, where the documentation stops short.
What "the instrument controls the radio" means
The link is a serial one, and the instrument speaks a documented set of commands to the radio. Depending on the model, it can read and write:
- Active frequency and standby frequency
- Volume, squelch and VOX level
- Switch — swap active and standby
- Dual — dual watch on or off
- Frequency spacing — 25 kHz or 8.33 kHz
The instrument also reports back: whenever a radio parameter changes, whether you changed it on the instrument or on the radio's own knob, the instrument emits the new state. A connected PDA or tablet therefore knows the tuned frequency without asking.
This is also why the navigation page can hand an airfield frequency to the radio. The setting is Send APT freq. under Setup › NMEA: with it on, navigating to an airport puts that airport's frequency into the radio's standby.
Supported radios on the LX heads
| Radio | Active / standby | Volume | Squelch | VOX | Switch | Dual |
|---|---|---|---|---|---|---|
| KRT2 | Yes | Yes | Yes | Yes | Yes | Yes |
| ATR833 | Yes | Yes | Yes | — | Yes | Yes |
| Becker | Yes | — | — | — | Yes | — |
| Trig | Yes | — | — | — | Yes | — |
| AIR avionics (ACD) | Yes | — | — | — | Yes | — |
Two things to read out of that table. First, the KRT2 is the fullest integration — everything the protocol defines except frequency spacing, it does. Second, if all you need is "put the tower frequency in standby and let me swap to it", every radio on the list will do that.
Becker radios need a special radio bridge for communication with the Era and LX 10K. Frequency spacing is defined in the protocol but is not listed against any of these radios — set spacing on the radio itself.
Wiring it: the user port
The radio connects to the instrument's user port, using RS232. That port is bidirectional NMEA, the same one a wired Oudie uses — which is why you cannot have both on it at once.
The physical wiring, connector by connector, belongs to the installation manual for your instrument and the manual for your radio. Get the cable made up properly rather than improvised: this is a connector that vibration works on, and a radio that drops out only in turbulence is a miserable thing to diagnose in the air. There are ready-made options — the vario to KRT2 radio cable, the KBS1 cable set and the KRT2 plug adapter.
Setting it up in the menu
- Go to
Setup › NMEA › Radioand select your radio from the list. The wrong model on a correct cable produces silence, not an error. - Tick
Send APT freq.if you want the instrument to push the selected airport's frequency to the radio. - Check the user port baud rate while you are in that menu. The default is
38400.
There is one consequence worth knowing before you turn it on: with Radio set to On, all other outputs on the user port are disabled. They are still present over Bluetooth. So an installation with a radio on the cable and a PDA on Bluetooth works; a radio and a PDA both on the cable does not.
NAVIA: a radio is a declared device
NAVIA Core Pro has no dedicated radio pins. It has five software-assigned RS232 ports, and you tell the system what is on which one under Airplane Settings › Devices. The recommended standard installation puts radio data on RS232 3, and takes radio power from POWER_OUT1 so the system can switch the radio off in flight if it must. Power can equally come from a Vertical Power VP-X system or from the avionics bus through its own breaker.
Documented integrations, with wiring tables in the installation manual for each:
- TQ KRT2 — full data control and centralised audio. Also available as the remote-controlled KRT2 RC.
- Air Avionics Air Control Display — a control head rather than a radio. NAVIA sends commands to the ACD, which routes them to whichever COM or transponder hardware sits behind it. It can also carry a built-in certified altimeter.
- Trig TY91, TY92, TY96 and TY97 — same quick-release D-Sub style as the NAVIA family, so the physical work is identical to the rest of the installation. The radio's Power ON pin must be grounded for it to come up with the system.
- Becker GT6201, AR6201 and AR62xx — these use RS422 natively. An external RS232-to-RS422 converter is mandatory. Do not skip it and hope.
- Funke ATR833 and ATR833S — identical pinout for this purpose.
Beyond frequency management, NAVIA's offline voice assistant will tune the radio hands-free. An AIR COM aircraft radio is driven through the Air Control Display — the ACD is the documented integration, not the radio itself.
Audio, microphone and PTT are a separate job
Do not treat the data cable as the whole installation. The serial link carries control; audio, microphone and push-to-talk are wired separately, and the installation manual's audio section governs them. Three rules the NAVIA documentation is firm about:
- Prefer the radio's unamplified headset output over its amplified speaker output. It gives a cleaner line-level signal and avoids over-driving the input.
- Use shielded wire for all audio and microphone lines, and ground the shield at one end only — the NAVIA end.
- Wire microphones to the standard electret inputs, not the dynamic ones.
Power: the load that finds a weak bus
A transmitting radio is the largest single current draw in most panels, and it appears as a step load rather than a steady one. If your supply run, your connectors or your battery are marginal, the transmit key is what will reveal it — usually as an instrument that reboots exactly when you talk. Measure bus voltage at the last device on the chain, during a transmit, at the end of a flying day.
That is the honest test, and it is covered properly in the academy article on power systems.
Transponders: what is documented, and what is not
This section is short on purpose.
On NAVIA, a transponder is a declared device like any other: assign it to an RS232 port under Airplane Settings › Devices, and power it from a controlled output if you want the system to manage it. The Air Control Display can sit in front of a remote transponder and be driven by NAVIA. We sell the VT-01 Mode-S transponder and the KTX2; ask us about a NAVIA installation before buying.
On the Era, LX 10K and Colibri X, there is a Transponder setting in Setup › NMEA. Switching it on drops the FLARM port to 9600 baud and transmits GPS data on that port — a transponder needs a position source, and that is how it gets one. Beyond that setting, there is no published transponder wiring procedure for these instruments. We are not going to invent one here. Ask your dealer, or ask us with your instrument and transponder model to hand.
When it does not work
| Symptom | Cause | Fix |
|---|---|---|
| Radio ignores the instrument entirely | Wrong model selected in Setup › NMEA › Radio | Select the actual radio; a near-match is a non-match |
| Frequencies work, volume and squelch do not | That radio only supports part of the command set | Check the table above — this is expected, not a fault |
| Airfield frequency never reaches standby | Send APT freq. not ticked | Tick it in Setup › NMEA |
| PDA on the user port went dead when the radio was added | Radio on the user port disables the port's other outputs | Move the PDA to Bluetooth |
| Becker radio silent on NAVIA | RS422, no converter fitted | Fit the RS232-to-RS422 converter |
| Instrument reboots on transmit | Voltage drop under the transmit load | Measure at the last device during a transmit; fix the supply, not the instrument |
| Radio does not power up with the system (Trig) | Power ON pin not grounded | Ground it per the installation manual |
Where to go next
- What the commands look like on the wire: LX NMEA sentences for developers.
- The other thing that hangs off the user port: connecting XCSoar, Oudie and other PDAs.
- Theory: transponders and ADS-B and power systems.
- Installation manuals for your instrument and radio: the download centre.
- Anything that does not match this article: your dealer first, then LX navigation.