一、什么是ModBus通讯协议

　　Modbus协议是应用于电子控制器上的一种通用语言，此协议支持传统的RS-232、RS-422、RS-485和以太网设备。

　　ModBus功能码

二、ModBus通讯协议的.Net实现

　　　　推荐一个github的开源实现类库：NModBus，github地址：https://github.com/NModbus/NModbus。针对TCP、UDP、RTU等的ModBus通讯方式都有实现。

　　　　读写方法：

　　　　每个方法都有各自的异步方法实现。

三、NModBus Master 示例

private static void Main(string[] args)
        {
            try
            {
                ModbusSerialRtuMasterWriteRegisters();
                ModbusTcpMasterReadInputs();
            }
            catch (Exception e)
            {
                Console.WriteLine(e.Message);
            }

            Console.WriteLine("Press any key to continue...");
            Console.ReadKey();
        }

        /// 

        /// Simple Modbus serial RTU master write holding registers example.
        /// 
        public static void ModbusSerialRtuMasterWriteRegisters()
        {
            using (SerialPort port = new SerialPort("COM3"))
            {
                // configure serial port
                port.BaudRate = 9600;
                port.DataBits = 8;
                port.Parity = Parity.None;
                port.StopBits = StopBits.One;
                port.Open();

                var factory = new ModbusFactory();
                IModbusMaster master = factory.CreateRtuMaster(port);

                byte slaveId = 1;
                ushort startAddress = 100;
                ushort[] registers = new ushort[] { 1, 2, 3 };

                // write three registers
                master.WriteMultipleRegisters(slaveId, startAddress, registers);
            }
        }

        /// 

        ///     Simple Modbus TCP master read inputs example.
        /// 
        public static void ModbusTcpMasterReadInputs()
        {
            using (TcpClient client = new TcpClient("127.0.0.1", 502))
            {
                var factory = new ModbusFactory();
                IModbusMaster master = factory.CreateMaster(client);

                // read five input values
                ushort startAddress = 100;
                ushort numInputs = 5;
                bool[] inputs = master.ReadInputs(0, startAddress, numInputs);

                for (int i = 0; i < numInputs; i++)
                {
                    Console.WriteLine($"Input {(startAddress + i)}={(inputs[i] ? 1 : 0)}");
                }
            }

            // output: 
            // Input 100=0
            // Input 101=0
            // Input 102=0
            // Input 103=0
            // Input 104=0
        }

example: https://github.com/NModbus/NModbus/tree/develop/Samples