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Some of the ExpressLRS TX modules include an additional ESP8285 chip, which lets us communicate wirelessly with other ESP8285 enabled devices using a protocol called espnow. We call this chip the "TX-Backpack". The aim of the TX-Backpack is to allow wireless communication between ExpressLRS, and other FPV related devices for command and control, or for querying config.
A prime use case is your video receiver module (or VRX). Currently there aren't many VRX modules that have an ESP8285 built in to allow them to communicate with ExpressLRS, so in most cases you need to add your own. A small ESP based receiver can be "piggybacked" onto your VRX module, which allows ExpressLRS to control the band and channel that your goggles are set to. We call this device the "VRX-Backpack".
Check the supported hardware page to make sure you have everything you need for the integration: Supported Hardware
Wiring guides for adding the VRX-Backpack to your module: Wiring Guides
Flashing guides for getting the firmware onto the VRX-Backpack and TX-Backpack: Flashing Firmware
- Solid = On and waiting to receive packets.
- Rapid continuous flashing = WiFi has turned on. Connect to the "ExpressLRS Backpack" access point and open http://10.0.0.1 in your browser. From here firmware can be uploaded.
- Double flash and pause = Binding mode. Press the Bind button in your ELRS TX LUA script to bind the TX backpack to the VRx Backpack.
- Hold down during power up to force boot mode. Firmware can now be flashed with a FTDI dongle.
- Single press during Normal or Binding mode will restart the Backpack into WiFi mode.
- Single press during WiFi mode will restart the Backpack into normal mode.
Just like the ELRS receiver, a binding phrase can be used when flashing Backpacks. When used your TX and VRx Backpacks will instantly be bound.
-DMY_BINDING_PHRASE="default ExpressLRS binding phrase"
To enter Binding mode manually when a binding phrase has not been defined, power cycle the VRx Backpack 3 times in a row. Turning on and off only needs to be done within 30s, and not 2s like the ELRS receivers. This provides enough time to power cycle your goggles and VRx module gracefully.
Using the button to enter and exit WiFi mode can also be used to power cycle the Backpack 3 times and enter Binding mode.
The LED will now double flash to indicate binding mode. The Binding button in LUA can now be pressed. LUA will tell the Tx Backpack to bind to you VRx Backpack. The LED should now stop blinking and go solid.
You are now bound and can use the VTx Administrator to send settings to the VTx and VRx.
- When in Binding mode and no bind packets have been received within 120s, the VRx Backpack will switch to WiFi mode. This provides a method of entering WiFi mode when a VRx Backpack is not bound and the button can not be used.