Circuit

Auto-switching UART-to-RS485 converter:

                   VCC ---------------+
                                      |
                              +-------x-------+
        (PIN27)    RXD <------| RX            |
                              |      UART    B|----------<> B
        (PIN26)    TXD ------>| TX    TO      |
ESP32                         |     RS-485    |     RS485 bus side
                              |               |
                              |              A|----------<> A
                              |               |
                              +-------x-------+
                                      |
                                     GND

Manual switching UART-to-RS485 converter:

                   VCC ---------------+
                                      |
                              +-------x-------+
        (PIN27)    RXD <------| R             |
                              |      UART    B|----------<> B
        (PIN26)    TXD ------>| D     TO      |
ESP32                         |     RS-485    |     RS485 bus side
        (PIN25)    RTS --+--->| DE            |
                         |    |              A|----------<> A
                         +----| /RE           |
                              +-------x-------+
                                      |
                                     GND

NB: ESP32 pins are configurable at compilation time.

Modbus

Device acts as Modbus Master, regurlaly polling a list of registers from a Modbus Slave.

List of Modbus parameters (in platformio.ini file):

• modbus_rxd: RX/R pin number (default: 27)
• modbus_txd: TX/D pin number (default: 26)
• modbus_rts: DE/RE pin number (default: 25). If you don't need RTS, use NOT_A_PIN value.
• modbus_baudrate (default: 9600)
• modbus_unit: Modbus Slave ID (default: 10);
• modbus_retries: if a Modbus request fails, number of retries before passing to the next register (default: 2)
• modbus_scanrate: the device will attempt to poll the slave every XX seconds (default: 30)

Registers list is defined by the array registers[] in src/modbus_registers.h. A very simple example would be:

const modbus_register_t registers[] = {
    { 123, MODBUS_TYPE_HOLDING, REGISTER_TYPE_U16, "value_123" },
    { 124, MODBUS_TYPE_HOLDING, REGISTER_TYPE_U16, "value_124" }
};

Where:

• 123 and 124 are register address to read,
• MODBUS_TYPE_HOLDING is the Modbus object type to read,
• REGISTER_TYPE_U16 is the expected format of the returned value,
• value_123 and value_124 are the name in the JSON MQTT message

Supported Modbus objects:

• HOLDING type is supported and has been tested
• INPUT, COIL, DISCRETE and COUNT has not been tested but should work

Supported returned Value:

• REGISTER_TYPE_U16: unsigned 16-bit integer
• REGISTER_TYPE_BITFIELD: a sequence of 16 (or less) single bits; the first item maps the least significant bit
• REGISTER_TYPE_DIEMATIC_ONE_DECIMAL: a specific De-Dietrich signed decimal implementation
• REGISTER_TYPE_DEBUG: hexadecimal value only visible in INFO logs (not sent in MQTT message)
• other types are not supported (TODO src/modbus_base.cpp:readModbusRegisterToJson)

MQTT

MQTT parameters are passed through environment variables:

• PIO_MQTT_HOST_IP: MQTT broker IPv4
• PIO_MQTT_PORT: MQTT broker port
• PIO_MQTT_TOPIC: root prefix of the published topic

With PIO_MQTT_TOPIC=MyTopic, based on the register list example above, the published MQTT message will be:

Topic: MyTopic/ESP-MM-ABCDEF012345/data
Message: {"value_123":0,"value_124":65536}

Where ABCDEF012345 is the ESP unique Chip ID.

Compilation

git clone https://github.com/gmasse/esp-modbus-mqtt.git
cd esp-modbus-mqtt
python3 -m venv venv
source venv/bin/activate
pip install -U platformio
hash -r
cp platformio.ini.dist platformio.ini
(edit platformio.ini if needed)
export PIO_FIRMWARE_URL="http://domain.com/firmware.bin"
export PIO_MQTT_HOST_IP=11.22.33.44
export PIO_MQTT_PORT=1883
export PIO_MQTT_TOPIC=mytopic
platformio run

Built firmware will be at .pio/build/fm-devkit/firmware.bin You can upload to ESP with: platformio run upload

TODO

• Configuration (Wifi credentials) Reset
• Factory Firmware (https://docs.espressif.com/projects/esp-idf/en/latest/api-guides/bootloader.html)
• Secure Boot
• Moving to ESP-IDF Framework
• Log level update at runtime

FAQ

Passing environment variables via VS Code

Edit .vscode/settings.json and add the following lines:

  "terminal.integrated.env.osx": {
    "PIO_FIRMWARE_URL": "https://url/firmware.bin",
    "PIO_MQTT_HOST_IP": "11.22.33.44",
    "PIO_MQTT_PORT": "1883",
    "PIO_MQTT_TOPIC": "mytopic"
  },

Flashing firmware

cp .plateformio/packages/framework-arduinoespressif32/tools/sdk/bin/bootloader_dio_40m.bin .
cp .platformio/packages/framework-arduinoespressif32/tools/partitions/default.bin .

esptool.py --chip esp32 --port /dev/ttyUSB1 --baud 460800 \
 --before default_reset --after hard_reset write_flash -z \
 --flash_mode dio --flash_freq 40m --flash_size detect \
 0x1000 bootloader_dio_40m.bin 0x8000 default.bin 0x10000 firmware.bin