Radiomaster MT12 - ELRS Arming (Channel 5 / AUX1)

As these radios now come fairly well setup for basic use of throttle and steering out of the box, I’m going to start with something less obvious but massively important to new users and something which is not setup out of the box.

“Arming is an extremely important part of the performance of the control link.”

Arming the ELRS system. 

ELRS is not like most surface radio protocols, and EdgeTX is a very low level operating system that requires a bit of patience to get right. It's also capable of hosting several different protocols which need different attention. There are a few things specific to how ELRS operates that are important in the setup.

One thing that appears a lot in the ELRS guide is that arming on channel 5 (also named Aux1 on some radios and documentation) is really important to the performance and reliability of the radio link.

Why is Arming important? Well there’s a rundown from the ELRS website of what arming does and how it affects the radio link at the bottom of this page in the FAQ section:
https://www.expresslrs.org/software/switch-config/

It states that it basically improves performance and link stability. One key performance factor is that it fully enables the dynamic power mode, which I and others had been having trouble with (causing cars to runaway). 

Here are some key points:

"When IsArmed is enabled, these safeguards are applied:

- All "Button" inputs are disabled
- All "Joystick" (5-way buttons) are disabled
- Bump to Share is disabled
- VTX Admin is disabled
- Integrated VTX channel change is disabled

When IsArmed is enabled, these performance features are activated:

- Dynamic Power is fully enabled
- Race telemetry mode turns telemetry off
- Some thermal-based fan controls are enabled

Arming is an extremely important part of the performance of the control link. Please use Aux 1 / Chan 5 as indicated. Also keep in mind that for ExpressLRS, ~1000us is the disarmed state and ~2000us is the armed state."

That seems like evidence enough that this is important. So it needs to be set up as a priority to get the best out of the radio. But I’m going to start on a slight tangent…

In this Joshua Bardwell video he explains how to set a throttle override using a logical switch that is controlled by the physical switch SD on the base of the radio, next to the USB-C charging port. That function does two things on his setup: 

1. It overrides the throttle so it doesn’t respond while you’re handling the radio.

2. It indicates the state of the throttle by changing the LED on the base of the radio to either; red (off) or green (on).

The video should be set to start at exactly the right point where he starts the explanation:

Once that function is setup, I then wanted to add a third thing that I want this switch to do at the same time. That is to also send the ARM signal, from the transmitter, on channel 5. The arm signal is a simple 1 bit on or off. Set at 1000 is off and set at 2000 is on. Simple right?

Using L01 (as set up in the Bardwell video) as the source for mix 5 and output 5 I have set the output trims and min/max so that the correct, high/low signal is sent for arm states with the press of the button. Here’s a screenshot of how I have the channel 5 output set. You could probably also set the same output numbers with a curve.

So now when I enable my trigger/throttle input I also set my arm status to on, and the LED's on the base of the controller go green.

You can check the arm status by going to the ExpressLRS LUA script in the SYS menu. In there it will flash armed in the top left if it is armed and nothing if it’s not armed. You’ll also see here whether you have dynamic power set up or not. 

Early testing with my cars, running the Radiomaster ER3Ci and ER5Ci receivers, has shown this to be a reliable way of making dynamic power work as intended, without random runaways or failsafes going wrong! I’ll keep testing and running, and I’ll update if I ever get another runaway car!