Calibration Mode

Mike Shellim 4 Dec 2013
Updated 23 April 2017

Introduction to Calibration Mode

Calibration Mode is a special flight mode which you can add to any model. It's an essential aid for calibrating your servos. When Calibration Mode is active, all mixers and trims are disabled, allowing absolute end points and centres to be visualised directly.

heading image


How it works

CAL mode is implemented very simply, by adding a single REPL mix to each servo channel. The new mix must be the last:


Mix 1 ...

Mix 2 ...


Mix N: Src=[stick] wt=+/-100 Multiplex=REPL Trim=NO_TRIM

Looking at the extra mix:

When the last line is active (we'll see later how to activate it), OpenTx will pass the raw stick value in the range -100 to 100 directly to the Outputs, where it's scaled according to the Min/Max/Subtrim values. This results in the following behaviour:

How to integrate Calibration Mode in your setup

Step 1 - create extra mixes

For each of the servo channels, add a new mixer-line as the last line. Parameters as follows:

Src is the stick or switch to control the calibration on that servo. You can choose any convenient control - it need not be the same as the control used in flight. Typically, a single control will drive a pair of related surfaces. Here's a typical scheme for a sailplane:

Step 2. Activate using a switch

The CAL lines may be activated directly via a switch, or indirectly via a dedicated flight mode. The flight mode method has the advantage that the flight mode is displayed on the screen, but requires that you use FM1 (the highest priority mode). If this is inconvenient (maybe you already have flight modes assigned), then the direct switch approach is fine.

Whichever method you choose, I strongly recommend using a virtual latch - the last thing you want is to accidentally activate calibration mode in flight!

Step 3. Set servo rotation and mixer weights

To make the calibration procedure as intuitive as possible, set the DIRECTION of each servo so that:

The easiest way is to try it and see: go into the servos menu; for each servo, adjust SUBTRIM back and forth, and see which way the control surface is moving. If you reverse the direction of a channel, remember that effect of each mix will be reversed. To correct this, reverse the sign of the wt parameter of all the mixers affecting that channel.

If you've set the direction of rotation as suggested above, then the weights in the CAL mixes should all be set to +100. This will ensure that all surfaces will move in a consistent direction during calibration - this is particularly useful for surfaces, like ailerons and flaps, which have been paired up for calibration.


Below is a screenshot from OpenTx Companion, showing two aileron channels and the extra CAL lines. The 'R' at the beginning denotes a REPL directive.

C9X screenshot


For OpenTx 2.0, 2.1, 2.2

SF-down to activate CAL mode

Stick calibration

For correct operation of your radio, remember that your sticks must also be properly calibrated. This is achieved via the OpenTx hardware calibration menu (Long press Menu -> Page x 8