Do FrSky receivers emit different types of SBUS?

[Allan Bennett]

I've got two different flight controllers in mutlirotors, which require an SBUS signal from the receiver.  In both of them I've tried using an XSR receiver (2 different ones in fact) and they're not recognised.  So the controllers are now running satisfactorily with the SBUS output from an X8R in one instance, and from an RX6R in the other.

 

So, have I got two duff XSR receivers?  I know they can be switched between SBUS and CPPM mode, but mine are definitely in SBUS mode, as indicated by a solid blue LED after they're bound to the Tx (v2 LBT ACCST firmware all round, by the way), and I've tried them with and without the blue LED, just to check.  The three wires for the SBUS connection come from pin 1 (gnd), pin 2 (+5v) and pin 4 (SBUS out) in the mini connector, as per the diagram in the XSR instructions (pin 3 is SPort, pin 5 is SBUS in). 

 

So, other than two bad receivers, the only other thing I can think of is, could the SBUS from the XSR be a different protocol to that from the other receivers?

[MattyB]

This might help. Some FCs do not like the inverted output from Frsky RXs apparently...

 

https://oscarliang.com/uninverted-sbus-smart-port-frsky-receivers/

[Allan Bennett]

That's interesting Matt.  The boards in question are a Matek 722 and a Pixhawk 2.4.8, and I don't know what F1/F3/F4/F7 designation they would be. 

 

But they're working with my RX6R and X8R receivers respectively, so there must be something different about the SBUS that the XSRs are putting out.  Since I don't seem to be able to use them anyway, I might as well hack into the uninverted SBUS connections that are shown in your link.  If that works with the FCs, then it would seem to confirm that some FrSky receivers produce uninverted SBUS, and others inverted.

 

I'll let you know how I get on.  Thanks a lot.

[Allan Bennett]

Just realised my receivers are RXSR, and they have an extra pad which Oscar Laing says is for uninverted SBUS.  So I soldered a wire there and used it for the connection to my Matek FC board, but it's still not recognised.  Not a problem, for the RX6R receiver is installed and working fine.  Just got to put a blanking plug over all those exposed PWM pins!

[Allan Bennett]

My Matek board is now running with the SBUS output from a no-pins X4R-SB receiver.  So that's three different receivers it does work with, and two RXSRs that it and my Pixhawk board don't work with.  Also, FrSky have now responded to a query I posted on their web site, saying that the SBUS from all their receivers is the same.

 

So, conclusion, I either have two damaged RXSR receivers, or I'm missing something incredibly simple in their setting up.  They both bind to my Tx, as evidenced by a green LED on the Rx and the RSSI indication on my Tx, and they both have blue LEDs on, which should indicated that they're in SBUS mode.

[Allan Bennett]

It works if I use the right firmware ?

 

When I initially went to the FrSky Downloads page I couldn't find the RXSR receiver listed in the ACCST receivers section (it still isn't there) so I went instead to the Flight Controllers section and used the RXSR-FC v2 ACCST firmware, thinking that the standard RXSR receiver is the one built into the integrated RXSR-FC Omnibus unit.

 

Today I went to a link that FrSky gave me for v1 firmware (they had suggested I roll back the firmware to see if it works), and found that it's actually the page for RXSR ACCESS and ACCST v2 firmware, as well as the legacy v1 files.  Installing RXSR_ACCST_2.1.0 (which the download page says is actually v2.1.1) has done the job ?

 

So the SBUS output from the four different FrSky receivers I've now tried is the same so far as my flight controller boards are concerned.

[Ron Gray]

Glad you got it sorted Allan.