RS-485 home sensors

The bridges and the Homey app store the broker address, so it must never change: use a DHCP reservation in the router, or a static IP. This guide writes it as <broker-ip>. Avoid addresses ending in .0 or .255: on a typical /24 network those are not usable host addresses.

Create /etc/mosquitto/conf.d/local.conf:

listener 1883
allow_anonymous false
password_file /etc/mosquitto/passwd

Leave the package's mosquitto.conf as is: it already enables persistence, so retained messages (link state, backlight, WiFi signal) survive a broker restart.

One for the bridges, one for Homey, so either can be revoked on its own.

# -c only for the first user: it creates the file
sudo mosquitto_passwd -c /etc/mosquitto/passwd wemos
sudo mosquitto_passwd /etc/mosquitto/passwd homey
sudo chown mosquitto:mosquitto /etc/mosquitto/passwd
sudo chmod 640 /etc/mosquitto/passwd
sudo systemctl restart mosquitto

If ufw is active, with your LAN subnet in place of <lan-subnet>. Never expose port 1883 to the internet: it is unencrypted.

sudo ufw allow from <lan-subnet> to any port 1883 proto tcp   # e.g. 192.168.1.0/24

sudo usermod -aG dialout $USER   # serial ports; log in again
curl -fsSL https://raw.githubusercontent.com/arduino/arduino-cli/master/install.sh | sh
arduino-cli config init
arduino-cli config set directories.user ~/arduino/arduino
arduino-cli config add board_manager.additional_urls \
  https://arduino.esp8266.com/stable/package_esp8266com_index.json
arduino-cli core update-index
arduino-cli core install arduino:avr@1.8.8 esp8266:esp8266@3.1.2
arduino-cli lib install "ModbusMaster@2.0.1" "PubSubClient@2.8"
# Homey CLI via Node.js 24
curl -o- https://raw.githubusercontent.com/nvm-sh/nvm/master/install.sh | bash
nvm install 24 && npm install -g homey

The Uno needs the fmalpartida "New LiquidCrystal" library, which is not in the Library Manager: copy ~/arduino/arduino/libraries/LCD from the old machine.

Use IDs 1–7, unique on the bus. Never use ID 0: it is the Modbus broadcast address and can never answer.

arduino-cli board list   # find the port, e.g. /dev/ttyACM0
cd ~/arduino/arduino/RadonSensorArduino_v2
arduino-cli compile --fqbn arduino:avr:uno .
arduino-cli upload -p /dev/ttyACM0 --fqbn arduino:avr:uno .

The firmware needs no configuration.

A to A, B to B, with a common ground between all nodes. Terminate the two physical ends of the bus with 120 Ω.

cd ~/arduino/arduino/WemosD1MiniRS485ModbusMqtt
cp secrets.example.h secrets.h   # gitignored, never commit it

BRIDGE_ID becomes the topic prefix rs485/<BRIDGE_ID>/. Don't change it after pairing in Homey. The profile sets the bus format: radon 8N1, power 8N2.

{7, DeviceType::Radon},      // radon profile
{10, DeviceType::Dtsu666},   // power profile
{11, DeviceType::Sdm120},

Only list connected devices: each one that doesn't answer blocks polling for about 4 s per register block. Radon sensors are polled every 60 s, meters every 10 s.

It ships in DL/T 645 mode, not Modbus. In the meter's menu (password 701, then Conn) set Addr to its ID and bAud to 9600. In Modbus mode it uses no parity and 2 stop bits.

UNKNOWN How to switch the protocol: the quick manual lists a "Protocol changing-over" register (0x002C) but doesn't show it in the menu. Check the full manual or the meter's menu before installing.

It ships at 2400 baud, ID 1. Hold its button for 3 s until the display shows -SET-, then write the holding registers over Modbus at its current 2400 baud, for example from the server with a USB-RS485 adapter and mbpoll:

Values are 32-bit floats, written with function 0x10. Power-cycle the meter afterwards. UNKNOWN The manual gives both "none" and "even" as the default parity; check the display.

Three DTSU666 details are not settled by its manual. Each is a one-line switch at the top of meters.cpp:

• UNKNOWN Function code: 0x03 assumed (kDtsuFunctionCode). If it times out, try 0x04.
• UNKNOWN Float word order: high word first assumed (kDtsuWordsSwapped). Nonsense values mean flip it.
• UNKNOWN Energy scaling: 1.0 assumed (kDtsuEnergyScale). Correct it if the kWh reading differs from the display.

Set MQTT_HOST in config.js to <broker-ip>, and create the gitignored env.json with the homey user. Keep it out of anything you share.

{
  "MQTT_USERNAME": "homey",
  "MQTT_PASSWORD": "<password>"
}