ESP8266 (ESP-12) Modbus TCP <-> RTU bridge

The simpliest (all in one file) Arduino / VSCode example. ESP8266 works as:

1. Modbus RTU master via UART (Software Serial port)
2. Modbus TCP server over WiFi

How it works

1. An external TCP client (such as a Windows PC application) sends a Modbus request to the ESP8266's local IP address.
2. The ESP8266 sens Modbus RTU requests vai UART (Serial port) to any Modbus slave device.
3. The ESP8266 receives a Modbus RTU response from the slave.
4. The ESP8266 transmits the response to the TCP client via WiFi.

Restrictions

The code supports the most commonly used functions:

• 0x03 READ HOLDING REGISTERS
• 0x06 WRITE HOLDING REGISTER
• 0x10 WRITE MULTIPLE HOLDING REGISTERS

Legacy features not supported: READ/WRITE COILS/DISCRETE INPUTS

How to use

• Visual Studio Code project in the VSCode project folder.
• Arduino IDE project in the Adruino project folder.

Enter your Wi-Fi credentials.

WiFi.begin("ssidname","password");

Replace the UART pins if necessary.

//  user defines
#define MB_UART_RX_PIN 13
#define MB_UART_TX_PIN 15

Enter the baud rate for the slave device.

swSerial.begin(9600, SWSERIAL_8N1, MB_UART_RX_PIN, MB_UART_TX_PIN);

How to find out the local Wi-Fi IP address of ESP8266.

Open the router's web page in a browser. View the list of clients.

How to debug

Use the built-in USB-COM serial port at the selected baud rate:

Serial.begin(115200);

Uncomment the line if you need debug Modbus messages:

#define MB_DEBUG