I installed a new but old EMAX Magnum Mini F4 V1 AIO in my quad today. The Rx is a Radiomaster ELRS RP3 which would normally use a pair of UARTS  to transmit and receive to the FC. The Magnum mini V1 only has pads for a 3 wire Rx. It actually has two sets of pads for a Rx. First I tried the pads labelled "Receiver" GND, 5V, SBUS/PPM but without success. Then I tried the pads labelled "SPECTRUM" GND, 3.3V, S and this is working just fine, the quad is flying. During my research I found a thread here where connecting an ELRS Rx with only three wires was discussed. There were some pros and cons about the reliability of connecting an ELRS without telemetry from the FC.
@SnowLeopardFPV said to put: set crsf_use_negotiated_baud = OFF in the CLI tab when doing this type of connection and I saw that it was already done in the CLI dump. So I'm happy I was able to get the quad flying but wonder why the "Receiver" GND, 5V, SBUS/PPM pads did not work. Sometimes I get things to work by just trying different combinations. The previously mentioned thread was helpful in telling me that the ELRS Rx connection was not necessary and I would just be giving up that type of telemetry while still having the OSD, but it also led me to believe that the SBUS pad should have worked? In any event using the set of pads with 3.3V is nice because I can have the Rx ON and the VTX OFF when connected to my computer via USB.
Cheers!

The SBUS pad has an inline hardware inverter on it so it's expecting an inverted signal which an ExpressLRS receiver doesn't output by default. You can get the ELRS firmware to output an inverted signal so it can be connected to an SBUS pad, but you need to switch on (tick) the RCVR_INVERT_TX option when flashing firmware to the receiver to allow that.

Oh that's a great explanation @SnowLeopardFPV. After reading that earlier thread I had looked at the ELRS LUA information in my transmitter for the RCVR_INVERT_TX as an option but now I see it is done by flashing firmware. I'm glad I was able to do it the way I did it but it is good to know this is another way and adds to my understanding.
Cheers!

I flashed some of my ELRS receivers to the newest version of ExpressLRS today and while doing so I remembered that the ExpressLRS devs moved the ability to switch on an inverted signal from the ExpressLRS Configurator to a setting in the web interface instead so that it can be changed on-the-fly at runtime without having to reflash firmware if you want to change that setting.

The RCVR_INVERT_TX setting got removed from the ExpressLRS Configurator starting with V1.6.0 and it got included as a setting in the ELRS firmware starting with version V3.3.0. So if you are running ExpressLRS firmware V3.3.0 or later, connect to the web interface of the receiver in a web browser and navigate to the "Model" tab where you will find a "Serial Protocol" setting. In the dropdown there are a number of different serial protocols you can select including some inverted ones that can be used when connecting an ELRS receiver to the inverted SBUS pad on an F4 FC.

ExpressLRS is s great way to go, init?

Posted yesterday:
https://youtu.be/-NBw3V9gSnU?si=mhwyrBcNlkik3Og6

Kind of cool that JB covered all of this yesterday. Exactly my problem.
I understand the concept of inverting the signal but not that I needed to do that to use ELRS with the SBUS pad. Or that I could get away with not using the Rx pad on the ELRS Rx.
So for anyone reading this thread and wondering I'll say using the SBUS pad and inverting the ELRS signal as shown above by @SnowLeopardFPV (or in JB's video) would have worked. Or typing in the CLI command. But since this EMAX FC had a separate set of Rx pads for a non inverted (ELRS) signal labelled "Spectrum" and I had a soldering iron in my hand that was the easier solution.
Note that both sets of signal pads on the FC use UART1, they can't both be used. The 3.3V pad could be used with either signal pad. And that newer versions of the EMAX FC have different pads and markings, including pads for access to another UART. My FC being V1 predates ELRS.

It looks like there was also an option to invert the signal via the web interface in ELRS firmware versions V3.0.0 - V3.2.1, but it was just a simple single "Invert TX Pin" tick box on the "Options" tab that inverted the CRSF protocol signal which was the only protocol that ELRS receivers supported up until until 3.3.0. I don't have any ELRS receivers on versions earlier than V3.3.0 so I can't check the web interface on those versions to get a screenshot without flashing back to an earlier version which I can't be bothered to do. but it's clearly shown in JB's video at time position 8:20. For versions of ELRS prior to V3.0.0 you have to set the inverted signal option during the firmware flashing phase in the ExpressLRS Configurator as shown in post #2 of this thread, making sure you use version 1.5.10 of the configurator which still contains the RCVR_INVERT_TX option in the UI.

Before we put this to bed I gotta ask a FC specific question. Do you think 3.3V is enough for the Rx? I mean it works, and I have flown it, but Radiomaster's website doesn't give a voltage range, it just says a "5V working voltage" for the RP1/2/3 series of Rx. I have the twin antenna RP3 here but the RP2 for instance is clearly designed for 1s operation which is not 5V. My choice of the 3.3V pad allows the Rx to function from USB (like the 4.5V pad typically seen on todays FC's) and the 5V pad does not, but I could use either...

The "cool quad bro" foto...

I couldn't find anything about a safe voltage range for the Radiomaster RP series of ELRS Rx. I know it works with 3.3V and that on a 1s whoop it can operate below that. But I don't know if that affects its transmission output and this 3" quad can cover a lot of ground fast. So I switched back to the 5v supply.

Sorry, I missed your last two posts, and specifically the question about the input voltage for the RadioMaster RP3 receiver. Other than some Spektrum receivers which can take a 3.3V input, nearly all other receivers require 5V to operate reliably, and in the manuals for the RadioMaster RP3 series it explicitly states the "Working Voltage" to be 5V, so I personally wouldn't risk using a supply voltage below that.

1S capable FC's have an onboard step-up voltage regulator to bring the 1S LiPo voltage up to 5V for any external peripherals so that they will always receive a stable 5V supply even if the LiPo is producing less then 5V to the FC.

Putting it back on a 5V pad was the safest and most sensible option IMO.

FYI, the serial output mode can also be set using the ELRS lua:
https://www.expresslrs.org/quick-start/t.../#protocol

Thanks, much appreciated, I did not know "1S capable FC's have an onboard step-up voltage regulator" cool.I am glad I got away with it on 3.3v for those first few flights