I have a Pixracer with Copter-4.0.7 and some stuff connected to serial 5 and 6. However, I can’t find parameter SR5 and SR6 in the parameter list using Mission Planner.
I can change SR0, SR1, SR2 and SR3, but the rest is missing.
My understanding is that SR1 doesn’t necessarily refer to serial 1 but to the first mavlink stream. So if your 2nd and 3rd mavlink outputs are on serial 5 and 6 the SR2 and SR3 parameters would affect serial 5 and 6.
From amilcarlucas’s comment it sounds like the three mavlink output limit is going away in 4.1, which would be pretty nice.