Recently, many users have reported that some F4 flight control Uart 2 ports are wired according to the crsf receiver in the manual, and the ELRS receiver is always on and can not work.
After data inquiry and test, it is determined that for the receiver with ESP chip, if the TX interface is placed at low Vil Mode,
This is not a bug in the firmware.
This problem is well known that some FCs cause the receiver TX line to go low at power on which forces the ESP into bootloader mode, hence the solid LED.
This is not a failure of the flight control or the receiver, but the ELRS project was not popular during the development of this part of the flight control models, and this problem was not optimized. The chips of ELRS and black sheep receiver are different, so the same crsf connection black sheep receiver can be used normally. The subsequent launch of flight control will solve this problem.
There are a couple of potential fixes/workarounds.
In addition, if there is a need for this part of flight control to use ELRS receiver, try the following methods:
1. Power on repeatedly for several times (low efficiency)
2. Remove some components on the flight control (high risk)
3. Add a pull-up resistor to port 2 (reply from the developer)
You can try is to add a small value (300ohm-1kohm) pull up resistor (to 3.3V) to the receiver TX line.
4. Change the version of flight control firmware (this problem occurs in some firmware of version 4.2)
5. ELRS receiver is connected to port 1, which can be used normally (Troubleshooting perfectly)
More details,please check the link below: