How to connect receiver to flight control board

The UART recommended by the manufacturer or assigned to the receiver typically has a pull-down resistor to assist with SBUS (RX pad inverted), which causes the LED on ESP-based ExpressLRS receivers to stay constantly lit. Unless you are very familiar with your UART configuration, it's best to avoid using these UARTs.

I. Introduction

UART-based ExpressLRS receivers communicate with the flight controller using the CRSF serial protocol. This protocol uses two pins: one for Control Signals and another for Telemetry Signals. You can think of it as a two-lane highway with no opposing traffic.

Below is a typical ExpressLRS receiver pin layout, indicating which flight controller pads each pin should connect to:

The TX pin of the ExpressLRS receiver sends or transmits the control signals it receives from the radio to the flight controller. At the same time, the RX pin of the ExpressLRS receiver accepts or receives telemetry data from the flight controller (such as battery voltage, current consumption, GPS coordinates and/or aircraft attitude) to send back to the radio.

Important

Note: The TX pin on the receiver should connect to the RX pad on the flight controller, while the RX pin on the receiver should connect to the TX pad on the flight controller

What is UART?

UART is a pair of RX and TX pads on the flight controller. It's commonly referred to as the flight controller's UART port, where you can connect different peripherals such as GPS or receivers. One UART can only be occupied by one device and can only perform one function.

R3 and T3 belong to UART3; RX2 and TX2 belong to UART2. Flight controller manufacturers mark their UARTs differently. Some only use "R" and "T" followed by the UART number. Others use "RX" and "TX" followed by the UART number.

UART is the acronym for Universal Asynchronous Receiver/Transmitter

II. Connecting the Receiver

• GND or G pad should be connected to any ground pad on the flight controller (FC)
• VCC or 5 pad should be connected to any 5v (or 4v5) pad on the flight controller (FC)
• TX or T pad should be connected to the RX pad of the same UART number where the RX or R line is located on the flight controller (FC)
• RX or R pad should be connected to the TX pad of the same UART number where the TX or T line is located on the flight controller (FC)

Ensure your wiring has no short circuits and power on the receiver.

• It's best to power the flight controller with a LiPo to ensure the receiver works properly under flight-ready conditions.
• It's crucial to ensure your wiring has no short circuits before powering on the system.

Observe the receiver LED. It should have one of the following behaviors:

	LED Indicator	Status
	Slow flash 500ms on/off	Waiting for connection from transmitter
	Double flash then pause	Binding mode is active
	Fast flash 25ms on/off	WiFi mode has started. Begins after 60 seconds.
	Solid on	Bootloader mode is active

If the receiver uses an RGB LED (such as Foxeer LNA, BetaFPV SuperD, Happymodel EP Dual), the following can be observed:

LED Indicator	Status
Rainbow fade effect	Booting
Slow flash 500ms on/off	Waiting for connection from transmitter
Orange: Double flash, then pause	Binding mode enabled
Green heartbeat	WiFi mode has started. Begins after 60 seconds

If the receiver LED indicator doesn't light up at all, quickly disconnect the power and check your wiring.

If your receiver has an RGB LED (such as Foxeer LNA, BetaFPV SuperD, Happymodel EP Dual) and the receiver LED still doesn't light up after ensuring the connections are correct, this may mean the receiver is in Bootloader mode.

If your receiver LED lights up but is just solid on, it means it's in Bootloader mode, as shown in the table above.

If the receiver is in Bootloader mode, you can do the following:

1. Check if the Boot button on the receiver is pressed or damaged. If the button is pressed or depressed, remove the cause of the press or depression. Heatshrink can sometimes press the button on the receiver. If the button is damaged, remove the button or replace with a better receiver. Consider asking the seller for a replacement.
2. If the receiver doesn't have a Boot button but has a Boot pad, check if the Boot pad is not connected to the ground pad. If the boot pad is connected to the ground pad, disconnect or remove the connection.
3. Disconnect the receiver's RX and TX wires from the flight controller. Reconnect the power to your drone or aircraft. Observe the LED on the receiver again.

• If it starts to slowly flash (500ms flash pattern), you need to move the receiver wiring to a different UART.
• If another UART is occupied by another peripheral, disconnect it and swap positions with the receiver.
• If there are no other UARTs available, you can try connecting a pull-up resistor: Connect a resistor (300 ohm to 1k ohm value; the lower the value, the more effective) between the RX pad of this UART and a 5v or 3.3v pad, as follows:

4. If you are using any digital FPV system such as DJI FPV Air Unit, Caddx Vista or DJI O3 Air Unit, and you have all six wires connected to your flight controller, you will need to remove the SBUS/DJI HDL wire.

If your receiver is behaving normally (i.e., not in Bootloader mode), then you can power off your aircraft and proceed to the next step: Configuring the flight controller

—— This article is referenced from ExpressLRS Getting Started: https://www.expresslrs.org/quick-start/receivers/wiring-up/#connecting-a-receiver**