Swapped ELRS RP1 in for Crossfire Nano, stuck in DFU mode? Won't bind or update...

[visual flame]

Helping a friend out with a brand new GEPRC Mk5DC that came with Crossfire nano installed.
-Removed the nano, used existing FC connections to connect a brand new Radiomaster RP1
-Confirmed FC Tx goes to RP1 Rx and vice versa
-RP1 does power up but goes solid green immediately even with transmitter off
-Swapped out for a different brand new RP1 receiver, exact same results
-Was able to flash the RP1 via UART using an FTDI adapter (cleared out binding phrase)
-Both transmitter and RP1 have been updated to ELRS 3.3.2
-When the RP1 is connected to the FC, it will never enter binding mode (solid green led all the time)
-Interestingly, if I disconnect the Tx and Rx wires from the RP1 and leave the 5V/Gnd the receiver will start functioning normally (it will go into bind mode and I was able to bind it to the transmitter that way, however as soon as I reconnect Tx and Rx wires to the FC it gets stuck in DFU mode
-DFU solder pad is NOT bridged or connected to anything
-We didn't change any BF settings for the FC, using factory CRSF protocol and port settings

This has been driving me nuts, any help would be greatly appreciated!

[young lake]

you've already tried a different uart for the receiver?

[visual flame]

Haven't tried a different UART yet, that's next on my list.

[young lake]

crossfire receivers doesn't have the same MCU as expressLRS receivers..

esp mcu used by expressLRS receivers can be affected by pull-downs on the uart.. unlike the tbs receivers

https://www.expresslrs.org/quick-start/receivers/wiring-up/

[visual flame]

Interesting, thank you! I'll try a different UART and see how that works.

[frank nest]

I have a similar problem with iFlight ProTek25 - FC SucceX-D F411 25A Whoop AIO - swapped receiver from FrSky to BetaFPV 2.4 Nano Rx and now on every power up it stucks in DFU (solid blue). I can get it working fine by quick unplug/re-plug from UART port - it loads fine, binds with Tx and I see stick movements in Betaflight.
I tried to move ELRS module to another UART but for some reason it doesn't even power up (checked different combinations of pins - the same).

Has anyone resolved this issue not just by changing UART but with another workaround?

On troubleshooting page https://www.expresslrs.org/quick-start/troubleshooting/ it is suggested to add a resistor to the scheme:
"If there's no other UART, wire up a resistor with a value between 300 Ohm to 1k Ohm between a 3.3v pad and the FC's RX pad (where the TX from the receiver connects to)."
but without some advice I don't think I will be able to do this 🙂

[young lake]

have you tried with just 5v and gnd connected, rx and tx disconnected from the FC?

do you have a dji air unit on the drone?

[frank nest]

Thanks for replying - I gave it another try and discovered that other UARTs work normally only with battery connected, haven't checked this option before, what a noob I am) Changed UART, connected battery and ELRS module is working fine now (at least in Betaflight). Hope I won't need to solder additional resistance somewhere to FC)