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Backup RX Input

Backup RX Input lets a spare UART carry a second, independent RX receiver ("satellite") purely as a backup for the main RF link. If the main receiver's signal is lost, the flight controller takes over all RC channels -- including aux/mode switches, so arm state and flight-mode switching keep working -- from the backup port instead, bypassing the normal failsafe stages entirely. The moment the main link's signal comes back, control reverts to it automatically. Takeover and revert happen within the main receiver's own signal-loss detection window (up to ~100ms), not on a per-missed-frame basis -- see Behavior.

This is a cheap way to get a basic backup-receiver setup (e.g. a small FrSky/compatible satellite bound to a second transmitter module or a different protocol entirely) without needing a full second RF system.

The backup port speaks a single, configurable protocol, chosen from the Protocol dropdown on the Serial RX #2 box (see below): SBUS, FBUS, F.Port, F.Port2, Jeti EXBUS, or TBS CRSF (Crossfire/ELRS). The feature was originally SBUS-only in name too ("SBUS-In Fallback Receiver"); it's now built so more protocols can be added later without changing how it behaves or is configured, hence the more general name. By default (before a protocol is chosen) the port is reserved but not decoding anything.

Setting it up

  1. Wire an RX-capable satellite receiver to a spare UART, then assign that port the Serial Rx (Backup) function from the port function dropdown on the Configuration tab's serial ports list. It's named to pair with the main receiver's own Serial Rx option -- this is a second, independent one.
  2. On the Receiver tab, a Serial RX #2 box appears below the main Serial RX #1 box. Pick the satellite's protocol from its Protocol dropdown, then Save & Reboot.
  3. Bind the satellite as you would any receiver of that protocol. If it isn't decoding (the box's Link badge stays down), check wiring against the UART's pinout -- the box's Pin Swap and Inverted switches cover boards where the port's natural RX pin isn't the one that's wired, or where the signal needs the opposite electrical inversion from the protocol's default assumption; Half-Duplex covers satellites that only expose a single, shared signal wire for this protocol. All three are independent of the main receiver's own equivalent settings, since this is a different physical UART, and each protocol has its own natural wiring convention (SBUS is inverted by default; every other supported protocol isn't) -- OFF always means "this protocol's normal wiring," not "no inversion" in an absolute sense. Changing any of these needs Save & Reboot to take effect.
  4. Turn off telemetry on the backup receiver itself. This link is receive-only -- the flight controller never reads anything the backup receiver's own telemetry system might send, so this has no effect on the feature's flight-control behavior at all. It matters for a different reason: a second, independent telemetry stream is exactly what can confuse your radio's Lua scripts about which link is the real telemetry path back from the flight controller.
    • FrSky/compatible satellites: in the receiver's own RX options (on the transmitter, not the Configurator), turn telemetry off for this receiver.
    • ExpressLRS receivers: use the ELRS web configurator to disable telemetry on this receiver.
  5. Once bound and decoding, the Serial RX #2 box's Link and Active Source badges update live, and each entry in Channel Assignment grows a second, smaller meter showing the backup receiver's own live channel values -- refreshed several times a second, the same readout used for bench-testing below.

Behavior

  • Bypasses staged failsafe, but isn't sub-frame instant. This does not go through the normal Failsafe stage machine at all -- no failsafe_delay/failsafe_recovery_delay involved. It reacts to the same signal-presence detection the main receiver already uses for its own signal-loss tracking, which is bounded to roughly 100ms after the last good main-link frame (not "next frame"). That's a deliberate choice: a feature-specific faster threshold could false-trigger a takeover on a legitimately slower RX protocol's normal frame spacing. It's still far quicker than doing nothing here -- an un-caught channel would otherwise hold its last value for 300ms before failsafe even declares it failed.
  • Full channel takeover. All channels are taken from the backup port during fallback, not just roll/pitch/yaw/throttle -- aux switches (flight mode, arming, etc.) are driven from the backup stick exactly as they would be from the main receiver.
  • Auto-revert. As soon as the main receiver's signal is valid again (within that same ~100ms window), control reverts to it -- the backup does not latch for the rest of the flight.
  • Not a second failsafe system. If both the main link and the backup link are down, ordinary staged failsafe behavior (hold/land/cut, per the Failsafe tab) takes over exactly as it would without this feature. Backup RX input is a bridge for a single lost link, not a replacement for failsafe.
  • A Failsafe switch on an aux channel still invalidates control channels even while the backup link is otherwise healthy -- the switch is honored the same way regardless of which link is currently active.

Bench-testing before you fly

As with any failsafe-adjacent behavior, test this on the bench (props off) before relying on it in the air:

  1. With both the main receiver and the backup satellite bound and powered, confirm the Receiver tab's Serial RX #1 box shows its Active Source badge as Main RX, and moving the backup satellite's sticks has no effect on outputs.
  2. Power off (or walk the main receiver's transmitter out of range), and confirm control switches to the backup satellite within roughly 100ms, arm state is preserved, and aux switches on the backup radio work as expected.
  3. Restore the main link and confirm control reverts to it within that same window.
  4. Power off both receivers and confirm ordinary staged failsafe triggers as configured.