Controlling Input Throw, Trim and Reversal

Where the parameters starting with SERVOn_ control the servo outputs, the parameters starting with RCn_ control the input side, related to pilot stick movement.

For the 3.8 firmware and above the input side and the output side are completely separated, allowing you to have completely different PWM ranges, reversal and trim on your transmitter input as compared to the servo output.

In addition to the RCn_ parameters for RC input range, trim and reversal, you also can choose the function of the first 4 input channels using the RCMAP_ parameters. The defaults are:

ParameterValue
RCMAP_ROLL1
RCMAP_PITCH2
RCMAP_THROTTLE3
RCMAP_YAW4

that means your first input channel will be roll input, the 2nd input channel will be pitch input, the 3rd input channel will be throttle input and the 4th input channel will be yaw input (which maps to rudder control in most flight modes).

The input range, trim and reversal are controlled by the RCn_MIN, RCn_MAX, RCn_TRIM and RCn_REVERSED parameters.

For example, if you have:

ParameterValue
RCMAP_ROLL1
RC1_MIN1100
RC1_MAX1900
RC1_TRIM1500
RC1_REVERSED0

then channel 1 input will be roll (mapping to aileron), with a minimum PWM of 1100, a maximum of 1900 and a trim of 1500. The RC1_REVERSED value of zero means it is not reversed.

Meaning of Reversed

For RC inputs, the following table shows the standard meanings of the REVERSED parameter for each input type.

InputREVERSED=0REVERSED=1
Roll Larger PWM values mean roll to the right Larger PWM values mean roll to the left
Pitch Larger PWM values mean pitch up Larger PWM values mean pitch down
Throttle Larger PWM values mean more throttle Larger PWM values mean less throttle
Yaw Larger PWM values mean yaw right Larger PWM values mean yaw left

Throttle Reversal

It is very unusual to have a reversed throttle input. In most cases it means your transmitter is setup incorrectly. Having a reversed throttle output is much more common for internal combustion motors. In firmware version 3.8 and later you do not need to reverse throttle input just because your output is reversed (ie. you can set RC3_REVERSED separately from SERVO3_REVERSED).

If you do need to use a reversed throttle input then you need to be be very careful with your throttle failsafe settings. The plane parameter THR_FS_VALUE is normally the throttle value below which the autopilot will consider your transmitter to be in RC failsafe. If you have a reversed throttle input then that value will need to be a high value, and will be the value above which you are in RC failsafe.