If you are setting up an FrSky receiver (like an R-XSR or XSR) on your FPV drone, you will inevitably run into the choice between SmartPort (S.Port) and FPort. Understanding the difference between these two protocols is crucial for correctly wiring your flight controller and getting telemetry data (like battery voltage) to display on your radio transmitter.
What is Telemetry?
Telemetry is the data sent from the drone back to your radio controller. Without telemetry, your radio only sends control signals (sticks and switches) to the drone. With telemetry, your drone can tell your radio its battery voltage, GPS coordinates, and signal strength (RSSI), allowing your radio to warn you before you crash.
| Protocol | Wiring Requirement | Pros & Cons |
|---|---|---|
| SBUS + SmartPort | Requires 2 signal wires. SBUS goes to an RX pad for control. SmartPort goes to a TX pad for telemetry. | Pros: Standard on older receivers. Very stable. Cons: Requires two separate UARTs on the flight controller. Harder to wire. |
| FPort | Requires only 1 signal wire. This single wire handles both control and telemetry. It goes to a TX pad. | Pros: Frees up a UART. Cleaner wiring. Slightly lower latency. Cons: Requires flashing specific FPort firmware to the receiver. Can have inversion issues on F4 boards. |
The F4 Flight Controller Inversion Problem
FrSky protocols (SBUS, SmartPort, and FPort) use an “inverted” electrical signal. This causes a massive headache depending on your flight controller’s processor:
- F3 and F7 Flight Controllers: These processors have built-in hardware inverters on every UART. You can wire S.Port or FPort to any TX/RX pad, flip a switch in Betaflight (
set tlm_inverted = on/off), and it works perfectly. - F4 Flight Controllers: F4 processors cannot invert signals in software. They usually have one specific pad with a physical hardware inverter dedicated to SBUS. However, SmartPort and FPort are bidirectional and usually require a dedicated, physically inverted TX pad, which many F4 boards lack.
Step-by-Step Guide: Making the Switch to FPort
- Step 1: Flash Receiver Firmware. You must download the specific FPort firmware for your receiver model (e.g., R-XSR) from the FrSky website and flash it to the receiver via your Taranis/Radiomaster radio.
- Step 2: The Wiring. Solder the SmartPort wire (yes, the S.Port pad is used for FPort) on the receiver to an available TX pad on your Flight Controller (e.g., TX3). If using an F4 board, use the “Uninverted S.Port” pad on the receiver if it has one.
- Step 3: Betaflight Configuration. Go to the Ports tab. Enable Serial RX on the UART you soldered to (e.g., UART 3). Do NOT enable telemetry on this screen. Save and reboot.
- Step 4: Betaflight Receiver Tab. In the Configuration tab (or Receiver tab in BF 4.3+), set the Receiver Mode to Serial-based receiver. Set the Serial Receiver Provider to FrSky FPort. Save and reboot.
- Step 5: CLI Tweaks (If it doesn’t work). If you have no stick movement, go to the Betaflight CLI and experiment with the inversion toggles. Type
set serialrx_inverted = on(or off), andset serialrx_halfduplex = on(or off). Typesaveafter each change until the sticks respond.
Visual Setup Guide
Ditch the Inversion Headaches
Struggling with FrSky firmware flashing and F4 inversion issues? The modern FPV industry has largely moved on to simpler, longer-range protocols that don’t suffer from these hardware quirks. Upgrade your radio link to ExpressLRS (ELRS). At UAVMODEL, we stock premium ELRS receivers that use standard, uninverted CRSF protocol—just 4 wires to any UART, and it works perfectly every time. Shop our ELRS selection and say goodbye to failsafes.