4、通信协议基础：Modbus RTU/TCP、DNP3、IEC 104、OPC DA/UA协议原理与对比

做SCADA运维这些年，我打交道最多的就是通信协议。说白了，协议就是设备之间沟通的“语言”。你想想看，一个站里几十上百台设备，PLC、RTU、智能仪表、变频器，要是各说各话，那不乱套了？

今天咱们就聊聊SCADA系统里最常见的四种协议：Modbus、DNP3、IEC 104，还有OPC。嗯，这里我得先说明一下，OPC严格来说不算纯粹的通信协议，它更像一个数据交换的“中间件”，但咱们运维人员经常把它和协议混在一起讨论，所以也放进来一起讲。

4.1 Modbus RTU/TCP：老当益壮的“通用语言”

Modbus是我入行接触的第一个协议。1979年Modicon公司搞出来的东西，到现在四十多年了，还在大量使用。为什么？因为它简单、稳定、够用。

4.1.1 Modbus RTU（串行通信）

RTU模式跑在RS-232或RS-485串行总线上。我个人习惯用RS-485，抗干扰能力强，传输距离能到1200米。

数据格式是这样的：

地址码（1字节） + 功能码（1字节） + 数据区（N字节） + CRC校验（2字节）

举个例子，读取从站地址为1的设备的保持寄存器，起始地址0，读取2个寄存器：

请求：01 03 00 00 00 02 C4 0B
响应：01 03 04 00 01 00 02 79 85

这里功能码03表示“读取保持寄存器”。响应里的04表示后面有4个字节数据，分别是00 01和00 02，也就是寄存器值1和2。

4.1.2 Modbus TCP（以太网通信）

Modbus TCP就是把RTU的报文封装到TCP/IP包里。端口号是502。去掉了CRC校验（因为TCP本身有校验），加了个MBAP报文头。

MBAP头（7字节） + 地址码（1字节） + 功能码（1字节） + 数据区（N字节）

MBAP头包含事务处理标识符、协议标识符（固定为0）、数据长度、单元标识符。

同样的读取操作，Modbus TCP的报文长这样：

请求：00 01 00 00 00 06 01 03 00 00 00 02
响应：00 01 00 00 00 07 01 03 04 00 01 00 02

4.2 DNP3：电力系统的“老大哥”

DNP3是电力行业用得最多的协议。它比Modbus复杂得多，但功能也强大得多。我记得第一次接触DNP3时，被它的三层结构和各种对象搞得头大。

DNP3的特点：

• 事件驱动：设备状态变化时主动上报，不用主站轮询
• 时间戳：每个数据点都带精确时间，方便事故分析
• 安全认证：支持SAv5安全认证，防止恶意攻击
• 多主站支持：一个从站可以同时和多个主站通信

DNP3的报文结构分三层：

物理层：数据链路层帧（起始字+长度+控制字+目的地址+源地址+CRC）
传输层：传输头（序列号+数据块）
应用层：应用头（应用控制字+功能码）+ 对象数据

举个例子，读取一个二进制输入点：

请求：C0 01 3C 02 06 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 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