Hey everyone, I am new to drones and in the middle of setting up my first quad.  Its a Geprc smart 35 that was purchased without a receiver so I could go ELRS right off the hop.  I'm using a Radiomaster Zorro which came with an internal ELRS module and 3 receivers (2 EP1's and 1 EP2).

I have updated the firmware on both the transmitter and the receiver.  The problem I'm having is that I can't seem to get the transmitter to bind with the receiver when the battery is connected.  Anytime you plug in the battery the receiver light turns solid green right away.

The weird part is that if the flight controller is hooked up to the USB cable but the battery is unplugged, It will blink green and bind with my transmitter as it should. It shows connection on the transmitter and I can see all my switches and sticks working as they should in betaflight. 

I'm hoping someone might be able to shed some light on what's going on. Is this a setting in betaflight that's not configured properly, or is it a hardware/installation issue?  I'm a bit lost where to start trouble shooting this other then de-soldering and trying one of the other receivers.

GepRC had some FAQs around this: https://geprc.com/product/geprc-elrs-nano-receiver/

Q:Why GEPRC ELRS rx is always on and can not work when powered it up?

A: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:
https://github.com/ExpressLRS/ExpressLRS/issues/655

Thanks kafie1980, I also found this in the ELRS guide that basically says the same thing you posted above.

https://www.expresslrs.org/2.0/quick-sta.../hmep2400/

Looks like I need to add a 300-1k ohm resister from the 3.3v pad to the FC RX wire/pad.

But instead of adding a 3.3v wire with an inline resistor to that particular UART, just try another UART? Isn’t this much easier to do?

What do you have on TX1 and RX1?

Rather than installing a physical external resistor, I wonder if the configurable PULLUP resources in Betaflight would work. Maybe they don't otherwise I would have expected the ExpressLRS documentation to have made mention of it. But it might still be worth a go to try and avoid having to unnecessarily add an inline resistor or re-solder to another UART.

Which UART is your ELRS receiver currently connected to on the FC?

My EP2 is connected to a Tx1 and SBUS pad, FC F405MK2, everything works without modifications.

Hey guys, just wanted to circle back and update this thread for anyone searching later on. I tried adding the resistor like mentioned in the ELRS installation guide, however, in my case that still didnt work. The guys over on the ELRS discord were fantastic though and helped me work through the issue.

I ended up having to swap the VTX RX/TX wires to UART2 from UART1, de-soldered the S-Bus wire, and then re-wire the receiver to UART1.

Everything now works as it should and I was able to do a hover test in the garage last night. Almost ready for its maiden voyage!

-Ryan

Glad you got it sorted. Happy flying.

I did have my doubts about the inline pullup resistor method on UART 2 which is why rewiring the receiver to UART 1 was my primary advice. 

Happy flying.

(23-May-2022, 01:45 AM)kafie1980 Wrote: Glad you got it sorted. Happy flying.

I did have my doubts about the inline pullup resistor method on UART 2 which is why rewiring the receiver to UART 1 was my primary advice. 

Happy flying.

I was just hesitant about doing this as it was suppose to be a BNF setup and I was just going to add a receiver to go ELRS. In the end it was really my only option so I buckled down and just did it.  Seems to be working well now! 

-Ryan