本文已收錄至：開源 DotNetty 實現的 Modbus TCP/IP 協議

ModbusFunction 類圖如下：

如前文所述，所有請求/相應的 PDU 均繼承自 ModbusFunction，其子類傳入對應的 Function Code 並實現三個方法：

1. CalculateLength：Data 部分的長度(該方法也可以為屬性，但屬性沒有強制性，怕漏掉故改為抽象方法)
2. Decode：從緩沖區解析 Data
3. Encode：在傳輸前對 Data 編碼

實現舉例

每個 Function Code 均對應 ModbusFunction 的兩個子類：請求類和響應類，以 0x03(讀取保持寄存器值)為例：

請求類

請求報文 Data 說明：

public class ReadHoldingRegistersRequest : ModbusFunction
{
    public ushort StartingAddress { get; private set; }
    public ushort Quantity { get; private set; }

    public ReadHoldingRegistersRequest()
        : base((short)ModbusCommand.ReadHoldingRegisters)
    {

    }

    public ReadHoldingRegistersRequest(ushort startingAddress, ushort quantity)
        : base((short)ModbusCommand.ReadHoldingRegisters)
    {
        StartingAddress = startingAddress;
        Quantity = quantity;
    }

    public override int CalculateLength()
    {
        return 2 + 2; // StartingAddress Length + Quantity Length
    }
    public override void Decode(IByteBuffer buffer)
    {
        StartingAddress = buffer.ReadUnsignedShort();
        Quantity = buffer.ReadUnsignedShort();
    }
    public override IByteBuffer Encode()
    {
        IByteBuffer buffer = Unpooled.Buffer();
        buffer.WriteByte(FunctionCode);

        buffer.WriteUnsignedShort(StartingAddress);
        buffer.WriteUnsignedShort(Quantity);

        return buffer;
    }
}
 

響應類

響應報文 Data 說明：

public class ReadHoldingRegistersResponse : ModbusFunction
{
    private ushort byteCount;
    public ushort[] Registers { get; private set; }

    public ReadHoldingRegistersResponse()
        : base((short)ModbusCommand.ReadHoldingRegisters)
    {

    }

    public ReadHoldingRegistersResponse(ushort[] registers)
        : base((short)ModbusCommand.ReadHoldingRegisters)
    {
        Registers = registers;
        byteCount = (ushort)(registers.Length * 2);
    }

    public override int CalculateLength()
    {
        return 1 + byteCount;
    }

    public override void Decode(IByteBuffer buffer)
    {
        byteCount = buffer.ReadByte();
        Registers = new ushort[byteCount / 2];
        for (int i = 0; i < Registers.Length; i++)
        {
            Registers[i] = buffer.ReadUnsignedShort();
        }

    }

    public override IByteBuffer Encode()
    {
        IByteBuffer buffer = Unpooled.Buffer();
        buffer.WriteByte(FunctionCode);
        buffer.WriteByte(byteCount);

        foreach (var register in Registers)
        {
            buffer.WriteUnsignedShort(register);
        }

        return buffer;
    }
}
 

其中 ModbusCommand 為 Function Code 的枚舉：

enum ModbusCommand : short
{

    ReadCoils = 0x01,
    ReadDiscreteInputs = 0x02,
    ReadHoldingRegisters = 0x03,
    ReadInputRegisters = 0x04,
    WriteSingleCoil = 0x05,
    WriteSingleRegister = 0x06,

    WriteMultipleCoils = 0x0F,
    WriteMultipleRegisters = 0x10,

}

文中為方便請求類和響應類均直接繼承了 ModbusFunction，實際開發中請求類和響應類均沒有直接繼承 ModbusFunction，而是根據其他 Function Code 的 Data 進行再次抽象後繼承。

開源地址：modbus-tcp

DotNetty 實現 Modbus TCP 系列 (二) ModbusFunction 類圖及繼承舉例