AP_RCProtocol: Serial port SBUS input erratic

[JacksonUAS]

Bug report

Issue details

Putting inverted SBUS onto a serial port as per multiple Rx install and enabling FS_THR_ENABLE results in looping failsafe alarm. FS_THR_ENABLE disabled results in no failsafe.

Version
AC4.0.3

Platform
[ ] All
[ ] AntennaTracker
[ X ] Copter
[ ] Plane
[ ] Rover
[ ] Submarine

Airframe type
X8

Hardware type
Cube Black
Herelink (inverted SBUS onto Serial 5/CONS)

Logs
https://discuss.ardupilot.org/t/dual-rx-input-insanity-herelink-and-rfd900x-sbus-passthrough/54620/8

[JacksonUAS]

Transposed SBUS inputs, can confirm there is a bug with the handling of failsafe condition on inverted serial receiver.

[JacksonUAS]

Issue exists on cube orange without external SBUS inversion as well.

[Pedals2Paddles]

Based on the log provided, the problem appears to be AP processing the RC input from the serial port. The log shows the channels all bouncing between good and no signal. This of course is making the failsafe kick on and off. It is NOT a failsafe problem. The failsafe is just a symptom of the RC handling problem. The user's log is attached.

He has already tested using multiple different inverters and difference receivers. Everything looks fine on a scope. But AP shows the input bouncing all over the place. Given his multiple methods of testing to rule out a hardware issue, I'm going to tag this as a bug. Feature originally added here last year #12152.

Log attached from @JacksonUAS
serialRCinput.zip

More discussion here: https://discuss.ardupilot.org/t/dual-rx-input-insanity-herelink-and-rfd900x-sbus-passthrough/54620

[Pedals2Paddles]

@JacksonUAS I changed the title since this is a core RC protocol library problem that impacts all vehicles and builds, not just a copter failsafe issue.

[JacksonUAS]

Any updates on this? Is there any timeline I can convey to operators about when this will be investigated.

[sachman]

Any update on this? This seems like a very serious bug,