一、IEC104协议核心特性与应用场景

IEC104（IEC 60870-5-104）是电力系统中广泛使用的通信协议，基于TCP/IP实现主从站（SCADA与RTU/变电站设备）的实时数据交互‌。其核心功能包括：

1.四遥操作‌：

‌遥测‌（YC）：采集电压、电流等模拟量数据（如类型标识0x0D）‌。
遥信‌（YX）：监测开关状态等数字量信号（如M_SP_NA_1单点遥信）‌。
遥控‌（YK）：远程控制断路器分合闸（如C_SC_NA_1指令）‌。
遥调‌（YT）：参数调节（如变压器分接头调整）‌。

2.‌协议分层‌：

APCI‌（应用协议控制信息）：包含启动字符0x68、长度域及控制域（I/S/U帧标识）‌。
ASDU‌（应用服务数据单元）：承载业务数据（如公共地址、传送原因、信息体列表）‌。

二、编解码实现

（一）解码器

public class IEC104FrameDecoder extends LengthFieldBasedFrameDecoder {
    private static final int MAX_FRAME_LENGTH = 253;
    private static final int LENGTH_FIELD_OFFSET = 1;
    private static final int LENGTH_FIELD_LENGTH = 1;

    public IEC104FrameDecoder() {
        super(MAX_FRAME_LENGTH, LENGTH_FIELD_OFFSET, LENGTH_FIELD_LENGTH, 0, 0); // ‌:ml-citation{ref="1,2" data="citationList"}
    }

    @Override
    protected Object decode(ChannelHandlerContext ctx, ByteBuf in) throws Exception {
        // 校验启动字符
        if (in.getByte(in.readerIndex()) != 0x68) {
            ctx.close();
            throw new ProtocolException("Invalid start byte 0x" + 
                                        Integer.toHexString(in.getByte(in.readerIndex()))); // ‌:ml-citation{ref="2,5" data="citationList"}
        }

        // 动态分割报文
        ByteBuf frame = (ByteBuf) super.decode(ctx, in);
        if (frame == null) return null;

        // 解析APCI
        frame.readByte(); // 跳过0x68
        int apduLength = frame.readUnsignedByte();
        if (frame.readableBytes() < apduLength) {
            return null; // 等待完整报文‌:ml-citation{ref="2,5" data="citationList"}
        }

        ByteBuf apduContent = frame.readSlice(apduLength);
        return parseAPDU(apduContent);
    }

    private APDU parseAPDU(ByteBuf buf) {
        // 解析控制域（4字节）
        int ctrl1 = buf.readUnsignedByte();
        int ctrl2 = buf.readUnsignedByte();
        int ctrl3 = buf.readUnsignedByte();
        int ctrl4 = buf.readUnsignedByte();

        ControlFieldType type;
        int sendSeq = 0, recvSeq = 0;
        UFrameCommand uCommand = null;

        // I帧（低比特位为0）
        if ((ctrl1 & 0x01) == 0) { // ‌:ml-citation{ref="2,4" data="citationList"}
            type = ControlFieldType.I_FRAME;
            sendSeq = ((ctrl1 & 0xFE) >> 1) | (ctrl2 << 7);
            recvSeq = ((ctrl3 & 0xFE) >> 1) | (ctrl4 << 7);
        } 
            // S帧（低2比特为01）
        else if ((ctrl1 & 0x03) == 0x01) { // ‌:ml-citation{ref="2,4" data="citationList"}
            type = ControlFieldType.S_FRAME;
            recvSeq = ((ctrl3 & 0xFE) >> 1) | (ctrl4 << 7);
        } 
            // U帧（低2比特为11）
        else { // ‌:ml-citation{ref="2,4" data="citationList"}
            type = ControlFieldType.U_FRAME;
            uCommand = UFrameCommand.fromBytes(ctrl1, ctrl2);
        }

        // 解析ASDU（仅I帧携带数据）
        ASDU asdu = null;
        if (type == ControlFieldType.I_FRAME && buf.readableBytes() >= 6) {
            asdu = new ASDU();
            asdu.setTypeId(buf.readUnsignedByte()); // 类型标识‌:ml-citation{ref="2,3" data="citationList"}
            asdu.setVariableStruct(buf.readUnsignedByte()); // 可变结构限定词‌:ml-citation{ref="2,3" data="citationList"}
            asdu.setCause(buf.readUnsignedShortLE()); // 传送原因（小端）‌:ml-citation{ref="2,5" data="citationList"}
            asdu.setCommonAddress(buf.readUnsignedShortLE()); // 公共地址‌:ml-citation{ref="2,5" data="citationList"}

            // 解析信息体（示例：遥测数据）
            int objCount = asdu.getVariableStruct() & 0x7F; // SQ=0时取低7位‌:ml-citation{ref="2,3" data="citationList"}
            for (int i = 0; i < objCount; i++) {
                InfoObject obj = new InfoObject();
                obj.setAddress(buf.readUnsignedMediumLE()); // 3字节地址小端‌:ml-citation{ref="2,5" data="citationList"}
                if (asdu.getTypeId() == 0x0D) { // 浮点型遥测
                    obj.setValue(buf.readFloatLE()); // ‌:ml-citation{ref="2,3" data="citationList"}
                }
                asdu.addInfoObject(obj);
            }
        }

        return new APDU(type, sendSeq, recvSeq, uCommand, asdu);
    }
}

(二) 编码器

public class IEC104Encoder extends MessageToByteEncoder {
    private final Recycler bufferRecycler = new Recycler<>() {
        @Override
        protected ByteBuf newObject(Handle handle) {
            return Unpooled.buffer(512);
        }
    };

    @Override
    protected void encode(ChannelHandlerContext ctx, APDU msg, ByteBuf out) {
        ByteBuf apciBuf = bufferRecycler.get();
        try {
            // 构建控制域
            switch (msg.getType()) { // ‌:ml-citation{ref="2,4" data="citationList"}
                case I_FRAME:
                    apciBuf.writeByte((msg.getSendSeq() << 1) & 0xFF);
                    apciBuf.writeByte((msg.getSendSeq() >> 7) & 0xFF);
                    apciBuf.writeByte((msg.getRecvSeq() << 1) & 0xFF);
                    apciBuf.writeByte((msg.getRecvSeq() >> 7) & 0xFF);
                    break;
                case S_FRAME:
                    apciBuf.writeByte(0x01);
                    apciBuf.writeByte(0x00);
                    apciBuf.writeByte((msg.getRecvSeq() << 1) & 0xFF);
                    apciBuf.writeByte((msg.getRecvSeq() >> 7) & 0xFF);
                    break;
                case U_FRAME:
                    apciBuf.writeByte(msg.getUCommand().getCtrlByte1());
                    apciBuf.writeByte(msg.getUCommand().getCtrlByte2());
                    apciBuf.writeZero(2); // 填充0x00
                    break;
            }

            // 构建ASDU（I帧）
            if (msg.getType() == ControlFieldType.I_FRAME && msg.getAsdu() != null) { 
                ASDU asdu = msg.getAsdu();
                apciBuf.writeByte(asdu.getTypeId()); // ‌:ml-citation{ref="2,3" data="citationList"}
                apciBuf.writeByte(asdu.getVariableStruct()); 
                apciBuf.writeShortLE(asdu.getCause()); // 小端写入‌:ml-citation{ref="2,5" data="citationList"}
                apciBuf.writeShortLE(asdu.getCommonAddress());

                // 信息体编码（示例：单点遥信）
                for (InfoObject obj : asdu.getInfoObjects()) {
                    apciBuf.writeMediumLE(obj.getAddress()); // 3字节小端‌:ml-citation{ref="2,5" data="citationList"}
                    if (asdu.getTypeId() == 0x01) { // 单点状态
                        apciBuf.writeByte(obj.getStatus() & 0x01); // ‌:ml-citation{ref="2,3" data="citationList"}
                    }
                }
            }

            // 构建完整APDU
            out.writeByte(0x68); // 启动字符‌:ml-citation{ref="2,5" data="citationList"}
            out.writeByte(apciBuf.readableBytes()); // 长度域‌:ml-citation{ref="2,5" data="citationList"}
            out.writeBytes(apciBuf);
        } finally {
            bufferRecycler.recycle(apciBuf);
        }
    }
}

(三) 实体类

// APDU实体类（对象池实现）
public class APDU implements Recyclable {
    private static final Recycler recycler = new Recycler<>() { /*...*/ };

    // 控制域字段
    private ControlFieldType type;
    private int sendSeq;
    private int recvSeq;
    private UFrameCommand uCommand;

    // ASDU字段
    private ASDU asdu;

}

// ASDU数据结构
public class ASDU {
    private int typeId;          // 类型标识‌:ml-citation{ref="2,3" data="citationList"}
    private int variableStruct;  // 可变结构限定词‌:ml-citation{ref="2,3" data="citationList"}
    private int cause;           // 传送原因（低字节在前）‌:ml-citation{ref="2,5" data="citationList"}
    private int commonAddress;   // 公共地址‌:ml-citation{ref="2,5" data="citationList"}
    private List infoObjects = new ArrayList<>();
}

// 信息体对象
public class InfoObject {
    private int address;  // 3字节地址‌:ml-citation{ref="2,5" data="citationList"}
    private float value;  // 遥测值
    private byte status;  // 遥信状态（0/1）‌:ml-citation{ref="2,3" data="citationList"}
}