Mike Shellim 25 July 2014
Last updated 18 June 2020
OpenTx is a uniquely flexible system. However if you've come from a brand like Futaba or Spektrum then you'll find the way of working a little unfamiliar. My aim with this article is to shed some light on how it works, and to enable you to design your own setups with confidence.
I'll cover the following topics:
Examples will be illustrated either with screenshots or as text boxes, as appropriate.
OpenTx has just eight programming menus, and each is completely generic. This concept of generic simplicity extends to the mixers. Just like Lego, simple elements are used to build powerful solutions.
But before you can use this power, you'll need to know some of the basics. So let's start at the beginning...
The core of OpenTx (as with any RC system) is the processing loop. This is a sequence of operations repeated several times a second - fast enough to provide a smooth response.
With each cycle, the position of all the controls are read, mixing is applied, and the commands for each channel are calculated. At the end of the cycle, the channel commands are passed to the RF module for transmission.
The three key processing steps are marked in blue. Each has an associated menu ('INPUTS', 'MIXERS' and 'OUTPUTS'). A change at one level propagates to levels lower down.
Like all computer programs, OpenTx needs some data to work with. In this case the data comes from your transmitter's sticks, trims, knobs and switches. A generic term for these is sources. Each source has a unique identifier, for example,
Each source carries a value between -100 and 100 according to its position. For sticks, left/down is negative; right/up positive. For switches, up is negative, down is positive. Zero corresponds to centre.
So for example,
Source values can be monitored in real time in the ANALOG INPUTS menu.
On most radios, a channel corresponds to a servo output. However in OpenTx, 'channel' has a looser meaning:
The OUTPUTS menu is where you configure the channel output.
Below are the Outputs for a simple RES sailplane. The first three channels have been named 'Rudder', 'Elev' and 'Spoilr'. The travels and centers are at their default values.
The OUTPUTS menu doesn't say how each channel is driven. For that, we need to define some mixers...
So far we've talked about channels in isolation. In order to do anything useful, we need some mixers. Mixers act as the 'wiring' from sources and channels; in other words, they define the control logic.
Unlike mixers in other radios, a mixer in OpenTX is a very simple construct:
A typical setup will involve anything from a couple of mixers (for an RE sailplane), to literally dozens of mixers (for a full house F3X ship).
OpenTX supports a maximum of 64 mixers. There are no built-in mixers, each must be explicitly defined by the user. The list of mixers which you've created is displayed in the MIXER menu.
Each mix processes the source value in some way, via mixer parameters like weight, offset, curves and so on.
Here's an example: a single mixer linking channel 1 to the aileron stick. Just the weight is specified, with all other parameters at their default values:
For simple mixes like these, just three columns are displayed:
A long press on any line takes you to the mixer editor where you can create or edit a mix. I won't go into the details here.
We'll now examine three key mixing scenarios - these will act as building blocks of your setup.
This is the simplest scenario. We've already looked at an example where the aileron stick drives CH1:
In this scenario, a single source drives more than one channel. In the example below, the aileron stick drives both CH1 and CH5. This is a popular configuration for dual ailerons:
This configuration emulates a Y-lead but is more flexible since each channel can be individually adjusted for direction, travel and centring via the OUTPUTS menu.
A single channel can also be driven by multiple sources simultaneously. A common example of this is a V-tail - each surface (channel) is driven by the rudder and elevator sticks:
Note the '+' sign against the Ele line - this indicates that the rudder and elevator inputs are added to produce the output. When using the add operator, the order of the mixers doesn't matter. Other operators like multiply and replace can also be used, and the order for these is important (we'll look at those later).
Now that we've covered the basics, let's go straight in and build a complete working system. The method I describe uses 6 steps and can be applied to any model, regardless of complexity.
To illustrate the method, we'll construct a 2-channel flying wing. While relatively simple, it shows the power of OpenTx's mixing system.
The first step is to make a list of the sticks and sliders used to control the model. These will be the sources for the mixers.
The sources for a flying wing are:
- Ail
- Ele
The second step is to assign the servo channels.
Our flying wing has two elevon servos. We'll assign these to CH1 and CH2. We can do this directly in the OUTPUTS menu:
Next, we identify the interactions between sticks and channels. An interaction is written as source->channel.
On a flying wing, the Ail stick must affect both elevon surfaces which we've assigned to channels 1 and 2. So,
- Ail -> CH1
- Ail -> CH2
Similarly, the interactions for Ele are:
- Ele -> CH1
- Ele -> CH2
Each line reads as "[source] affects [channel]"
The next step is purely mechanical: simply swap the left and right sides from Step 3.
- CH1 <- Ail
- CH2 <- Ail
- CH1 <- Ele
- CH2 <- Ele
So now, each line can reads as "[channel] is affected by [source]"
Now, group the mixer definitions by channel.
- CH1 <- Ail
- CH1 <- Ele
- CH2 <- Ail
- CH2 <- Ele
The mixers are now in the form for entering in the MIXER menu.
Finally, created the mixer definitions:
The '+=' means on each line shows that the mixes are additive.
So that's the basics of our flying wing setup. We're not quite finished though, as we haven't taken into account the different effects of the aileron and elevator sticks. We do his via the weight parameter.
There are various parameters you can set in the mixer editor, but the most important is weight. This determines the effect of the mix, between 0 and 100% in magnitude. The sign determines the direction. So
Let's illustrate this with our flying wing. Typically, roll commands require more elevon travel than pitch, so we might set a higher weight for the aileron mixes, say 90%, with say 30% for elevator. And since ailerons move in opposite directions in response to the stick, we make one of the weights negative:
This will work fine, but it can be improved, using Inputs.
Our flying wing example works fine, but there are a couple of shortcomings... First, in order to adjust a rate, we have to make the same adjustment in two places places. And secondly, navigating the MIXER menu can be tiresome, especially for complex models.
We can avoid both issues by using Inputs (introduced in OpenTx v. 2.0). An input is essentially just a regular source, but with weight and expo built in. Inputs are distinguished from regular sources by a leading '[I]'.
Inputs are managed in the INPUTS menu, which provides quick and easy access to rate and expo adjustments.
Using our flying wing example, we create a couple of inputs for the Ail and Ele sticks:
The [I]Ail and [I]Ele inputs behave exactly like their sources, except they implicitly have 90% weight and 10% expo.
We now amend the mixers to use the new inputs as sources. Since the weight has been set in the two new inputs, we reset the mixer weights to 100%. Where possible, always do your rate adjustmets in the inputs, not the mixers!
The use of inputs is optional. Sometimes it's better to use the raw source. Here are some guidelines:
Inputs are an important part of OpenTx, and you can read more about them here.
In this section we'll look in more detail at mixer processing. An understanding will be come in useful when debugging your setup.
OpenTX performs two sets of calculations. First, individual mixer line values:
Channel 1
mix_line_1 = Ail_stick_value x 90%
mix_line_2 = Ele_stick_value x 30%
Channel 2
mix_line_1 = Ail_stick_value x -90%
mix_line_2 = Ele_stick_value x 30%
Next, the mixer line values are aggregated to produce the mixer outputs. These represent the commanded positions for each channel:
CH1 Mixer output = (Ail_stick_val x 90%)+(Ele_stick_val x 30%)
CH2 Mixer output = (Ail_stick_val x -90%)+(Ele_stick_val x 30%)
The following table shows the mixer outputs for our flying wing example:
| S T I C K S | M I X E R O U T P U T S | ||
| Ail stick | Ele stick | CH1 | CH2 |
| (centre) 0 | (centre) 0 | 0 | 0 |
| (full right) 100% | (centre) 0 | 90+0=90 | -90+0=-90 |
| (full left) -100% | (centre) 0 | -90+0=-90 | 90+0=90 |
| (centre) 0 | (full forward) 100% | 0+30=30 | 0+30=30 |
| (half right) 50% | (half forward) 50% | 45+15=60 | -45+15=-30 |
| (full right) 100% | (full forward) 100% | 90+30=120 | -90+30=-60 |
The last line shows the effect of "stick in the corner": the mixer output for Channel 1 is 120. In fact, mixer outputs are clipped to +/- 100 in the OUTPUT stage. More on that later.
You can see the mixer outputs in real time in the mixers monitor (introduced in OpenTX 2.2). To activate it on the X9D, from the splash screen press {Page} until you get to the channels monitor, then press Enter.
OpenTx offers a number of mixer options. We've already looked at weight. We'll now investigate offset, expo, diff, functions and curves.
The offset parameter is the most important after weight. Offset is used to shift the mixer output up or down. Using offset in conjunction with weight, you can create mixes whose value varies within defined limits. This is useful for creating volume controls and compensation mixes.
Offset is applied after weight:
Mixer line value = (Source * weight) + Offset
Offset example 1: custom volume control
Suppose you want to create volume control (S1), with output varying from -50 to +90 as S1 is rotated. We can do this as follows (remembering that the value of S1 varies from -100 to +100):
MIXERS menu
CH10 (vol control)
Src = 'S1', wt=70%, offset=20
Offset example 2: motor-elevator compensation. Offsets are invaluable motor and crow compensation. In these mixes, the mix must be zero at one end of stick travel. To achieve this, weight and offset must have the same magnitude.
The example below shows a typical motor compensation mix. As the throttle stick is advanced, a corrective elevator input is applied up to a maximum of 40% (at full throttle).
MIXERS menu
CH2 (elevator)
Src=Thr, wt=20%, offset=20
In addition to weight and offset, OpenTx offers one additional parameter from the following list of choices:
The 'Curve' parameter provides the most granularity - it can implement any of the other options, however it requires the most data entry.
Sailplanes often employ aileron differential in order to reduce the travel of the downgoing aileron.
One source of confusion is the 'diff' parameter, as it's available in both the INPUT and MIXER menus. First, the incorrect way, using INPUTS menu:
INPUTS menu
[I1]Ail
Src=Ail, wt=90%, expo=10, diff=20
MIXERS menu
CH1 (right elevon)
Src = [I1], wt=100%
CH2 (left elevon)
Src = [I1], wt=-100%
Using the above code , the aileron stick will have a higher rate on one side than the other. This is not what we want! Instead, diff must be applied separately to each aileron mix:
INPUTS menu
[I1]Ail
Src=Ail, wt=90%, expo=10, diff=0
MIXERS menu
CH1 (right elevon)
Src = [I1], wt=100%, diff=20%
CH2 (left elevon)
Src = [I1], wt=-100%, diff=20%
To summarise: Do not specify diff at the Input level. Specify diff separately for each aileron mix!
By default, trim values are included in sources. You can exclude trims on a per-mixer basis by unchecking "Include Trim" in the mixer dialog.
For our flying wing example, aileron and elevator trims must always be active, so we'll use the default settings.
When you create a new mix, the initial settings are weight=100% and offset=0, diff/expo=0 and no curve. The source value is therefore passed through unchanged.
When calculating a mix line value, OpenTx first applies the trim value. Then it applies the function (diff, function, curve, expo), followed by weight, and finally adds the offset. Order may be important when combining operations especially where offsets are involved!
The OUTPUTS menu controls the final stage of processing
In the Output stage, OpenTX performs the following:
Let's look in more detail:
Clipping
Recall how mixer outputs are aggregated from individual mixer lines:
CHx_MixOutput = SUM (CHx_MixLine1, CHx_MixLine2, ...)
If several mixers are active, the mixer output may exceed the safe limits of the servo. To prevent this, the OUTPUTS layer clips the value so it lies in the range -100 to +100. Any clipping will manifest itself as deadband at extremes of stick movement.
Scaling and offset
After clipping, a scaling and offset are applied. The scaling and offset are defined by MIN/MAX/SUBTRIM and an optional curve. By setting the output parameters appropriately, you can compensate for differences between the left and right sides of the model.
Notes:
You can monitor channel outputs in real time, using the Channels monitor.
The three processing layers (Inputs, Mixers, and Outputs) each apply a weight or scaling to their inputs, and these values are cumulative. So final channel output is as follows (ignoring clipping):
OutputValue = SourceValue x Rateinput x Ratemix x Rateoutput
Clearly there is an infinite combination of rates which will produce the same movement at the servo. So where should you set them? Good practice is to adjust servo-related adjustments in the OUTPUTS, control rates in the INPUTS, and as little as possible in the MIXERS!
This is the procedure I recommend:
We've seen how mixer line values are aggregated to produce mixer outputs. So far, we've only used addition. In fact OpenTx also offers multiplication and replacement operations.
This is very useful. For example, multiplication is key to implementing in-flight adjusters, and 'replace' can be used to implement throttle safety switches.
The operator is specified in the mixers Multiplex parameter, and the options are 'ADD' (the default), 'REPL' and 'MULT'.
When aggregating mixer lines, OpenTx initially assigns a value of zero. Starting at the top line, it steps through each mix, and applying the result according to the multiplex parameter:
Note:
The following examples illustrate the effect of the various multiplex options. Note the importance of mixer order when using 'REPL' and 'MULT'.
Src = I1, op=ADD
Src = I2, op=ADD
result = I1 + I2
Src = I1, op=ADD
Src = I2, op=ADD
Src = I3, op=MULT
result = (I1 + I2) * I3
Src = I1, op=ADD
Src = I2, op=MULT
Src = I3, op=ADD
result = (I1 * I2) + I3
Src = I1, op=ADD
Src = I2, op=MULT (disabled)
Src = I3, op=ADD
result = I1 + I3
Src = I1, op=ADD
Src = I2, op=REPL
Src = I3, op=ADD
result = I2 + I3
Src = I1, op=ADD
Src = I2, op=REPL
Src = I3, op=MULT
result = I2 * I3
Src = I1, op=ADD (disabled)
result = 0
MAX is a special source which doesn't correspond to a physical control. Instead, MAX supplies a fixed value of +100. In conjunction with weight, it can be used to simulate the effect of a fixed stick position.
Src = MAX, wt =100 -- output = 100%
Src = MAX, wt=50 -- output = 50%
Src = MAX, wt=-100 -- output = -100%
Here's a simple example, showing a crude motor arming system. The motor is armed when SA is down.
MIXERS menu
CH7 (motor)
Src = Thr, wt=100%,
Src = MAX, wt=-100%, switch=!SA_down, multiplex = REPL
If you're familiar with F3X sailplanes, you may wish to look at interactions and mixes for F3X sailplanes. Such a scheme will normally be optimised by means of Inputs (described later in this article), GVARs and cascading mixers.
LapinFou has produced some useful data flow diagrams which describe the internal workings of OpenTx (Note: at the time of writing, there are one or two errors in the order of processing of curves).
