QNet-outstation

Features

All QNet Outstations run this Python application to handle connections to the MQTT Broker and interfacing with the onboard GPIO of the Raspberry Pi.

The MQTT broker disseminates messages between the server and Outstations.

There are two types of QNet model:

• Q-Lite – Is based on a Raspberry Pi Zero
• Q-Pro – Is based a Raspberry Pi 3B

Note

Q-Lite models must be configured to connect to the network wirelessly due to the lack of onboard physical network interface.

The Python application is agnostic of the hardware model and can be run on either type.

Installation and Setup

Physical GPIO Wiring

The default wiring of the GPIO devices is:

• RED_LED: Pin 4
• GREEN_LED: Pin 11
• BUTTON_PIN: Pin 16
• RELAY_SWITCH: Unassigned

These pins are chosen because of their proximity to a ground pin, allowing for a 2x1 2.54 mm dupont connector to be wired to each IO device.

The RELAY_SWITCH output is provisioned for future use and currently serves no purpose.

Initialisation Process

1. The application starts and the LED Controller and MQTT Client classes are both instantiated.
2. The device connects to the broker to announce itself as online.
3. The device enters the IDLE state and waits to receive commands from the server.

A system service should be configured to start the application automatically, after the network interface services (network.target) have started.

MQTT Message Structure

There are two message types (topics) that are used between Outstations and the Broker: status and command messages.

Status Messages

Status messages are always sent from an Outstation to the broker to announce connection or disconnections from the broker. Every Outstation on startup sends a connected status message and sets the LWT message to announce the disconnection on its behalf:

{
  "state": "IDLE",
  "online": true // or false
}

Command Messages

Command messages are used to inform the system of a change in state. These messages are bidirectional however the only state command an Outstation would send is the READY command.

{
  "state": "GO"
}

Additional Setup

Disabling Wi-Fi Power Save Mode

Some Raspberry Pi Wi-Fi models put the Wi-Fi chip into power-saving mode after a few seconds of inactivity which can lead to noticeable delays when using QNet.

To check if Power Save Mode is enabled:

iw wlan0 get power_save

To disable Power Save Mode:

sudo iw wlan0 set power_save off

This will improve latency across wireless QNet and make them more responsive.

A function is provided by default within the application to execute this command once the script starts.