Table of Contents

Modbus - Machine to machine communication protocol

Modbus communication protocol addresses architectures for distributed automation systems across multiple market segments. It can be serial port based or network based. This implementation is serial port based but can be adjusted to support networking easily.


Modbus specifications can be found on the Modbus organization website.


If you are using an ESP32, you must initialize the pins to use serial port like this:

// SerialPort COM2
Configuration.SetPinFunction(16, DeviceFunction.COM2_RX);
Configuration.SetPinFunction(17, DeviceFunction.COM2_TX);
Configuration.SetPinFunction(18, DeviceFunction.COM2_RTS);

With an ESP32, you can almost use any pin for any function. When using an STM32 or other MCU, the pins are usually fixed. Refer to the documentation of your MCU to understand which pin to use.

Modbus client

To initialize and use a Modbus client, you can do the following:

// Modbus Client
var client = new ModbusClient("COM3");
client.ReadTimeout = client.WriteTimeout = 2000;

client.WriteMultipleRegisters(2, 0x5, new ushort[] { 3, 5, 2, 3 });
client.Raw(2, FunctionCode.Diagnostics, new byte[] { 0x01, 0x01, 0x01, 0x01 });

var data1 = client.ReadHoldingRegisters(2, 0x7, 4);
var data2 = client.ReadCoils(2, 0x23, 2);

As shown in the sample, you have the possibility to write multiple registers, use raw function and also ready multiple registers.

Modbus server

A server implementation is also available:

// Modbus Server
var server = new ModbusServer(new Device(1), "COM2");
server.ReadTimeout = server.WriteTimeout = 2000;

This server implementation will allow you to act as a server. It can be extended.

Hardware implementation

When using Modbus with an RS485, you can for example use a MAX485 to make the interface between the traditional serial port and RS485: