DotNetty 实现 Modbus TCP 系列 (二) ModbusFunction 类图及继承举例

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ModbusFunction 类图如下：

如前文所述，所有请求/相应的 PDU 均继承自 ModbusFunction，其子类传入对应的 Function Code 并实现三个方法：

1. CalculateLength：Data 部分的长度(该方法也可以为属性，但属性没有强制性，怕漏掉故改为抽象方法)
2. Decode：从缓冲区解析 Data
3. Encode：在传输前对 Data 编码

实现举例

每个 Function Code 均对应 ModbusFunction 的两个子类：请求类和响应类，以 0x03(读取保持寄存器值)为例：

请求类

请求报文 Data 说明：

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;
    }
}

响应类

响应报文 Data 说明：

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 为 Function Code 的枚举：

enum ModbusCommand : short
{

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

    WriteMultipleCoils = 0x0F,
    WriteMultipleRegisters = 0x10,

}

文中为方便请求类和响应类均直接继承了 ModbusFunction，实际开发中请求类和响应类均没有直接继承 ModbusFunction，而是根据其他 Function Code 的 Data 进行再次抽象后继承。

开源地址：modbus-tcp