CAL Mode

Mike Shellim 4 Dec 2013
Updated 2 Dec 2020

Introduction to CAL Mode

In this article, I will explain how to add a special mode to your setup, for calibrating your servos. It can be added to any setup without affecting existing functionality.

heading image

What it does

Calibration is the process of adjusting your servo limits and centres. You do this in the OUTPUTS menu.

First, however, you have to generate mixer values of -100, +100 and zero as these represent the limits and centres. The CAL mode which I describe enables you to generate these values on demand, without affecting your existing settings.

Adding a CAL mode to your setup

The principle is quite simple: for each servo channel, you add a 'REPLACE' mix to the end of the existing mixers. The mixer is set up so that when it's active, it replaces all the mixes above it, passing through the stick value between -100 and +100.

In the example below, CH1 is calibrated via the elevator stick. The CAL line is actated in FM1 (but you could activate it via a safe switch, or a gesture, more on that later):


CH 01

[mix 1] ...

[mix 2] ...

[CAL mix] Src=Ele wt=100 Multiplex=REPL Trim=NO FlightMode=FM1

The actual calibration will be done in the OUTPUTS menu. For the example above:

More about the CAL mix

This section explains the setup in more detail.

Setting up the CAL mix lines

For each servo channel, add a mixer-line to the end of the mixer list:

Source is the stick/pot/lever to drive the servo during calibration. It need not be the same as the flight control. Specify the raw control only, for example Ele, Ail (do not select inputs like [I1]Ail as these include rates and expo).

It's a good idea to move related surfaces using a single control, so you can equalise their responses by eye. For example, here's a typical scheme for a sailplane:

Activating the CAL lines

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), so that it's available on demand. If this is inconvenient (maybe you already have flight modes assigned), then the direct switch approach is fine.

Safety note: whichever method is choosen, you must use a virtual switch - the last thing you want is to accidentally activate calibration mode in flight.

Setting output direction

To make the calibration procedure intuitive and consistent, set the DIRECTION in each output 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. (Remember that if you reverse an output, you will need to also reverse the mixers affecting that output.)


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 2.2, 2.3

SF-down to activate CAL mode

Hardware calibration

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