I am trying to connect a FRsky TFR4SB receiver to a pixhawk and I’m having problems. Here’s what I did and what it does. I bound the receiver. I set the receiver to Sbus 1 aka slow. I connect it to RC in on the pixhawk. When I turn on the pixhawk it powers up and the receiver shows it’s bound and powered on. However it seems like only ch 1 and 2 work. Am I connecting it right? One other interesting thing is channel 3 works [ I have a nitro so I can see the servo move ] when the pixhawk is booting up but after it’s done it stops working. I’m stumped. 
The FRsky says it’s compatible with the Futaba 7C TX I’m using.
Do I need one of these perhaps for the pixhawk to work with an s bus receiver?
ebay.com/itm/231237956016
re- ppm encoder… guess not. So I am at a loss.
“The port labeled “RC” can take normal PPM sum or Futaba S.Bus inputs and the port labeled “SB” can read RSSI our output S.Bus to servos”
Not only am I talking to myself I am answering myself too 
My bad… user error. I had flight mode channel set to 8. That was fine in my previous set up for reasons I bore you with but with the Sbus it had to be on ch7. So on boot up it went right into auto mode. Being a nitro engine and the pixhawk detects it is not moving at all it keeps the throttle [ ch3 ] at the low setting. Because I did not have throttle passthrough ‘on’ I could not use my TX to move ch 3. Once I set that in the params it worked fine.