ModBus/TCP v1.0 协议规范解读

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"MODBUS TCP v1.0.pdf" 是一份关于ModBus/TCP协议的开放规范，由Andy Swales代表Schneider Electric发布，版本号为1.0，日期为1999年3月29日。该文档详细阐述了ModBus/TCP的连接方式、数据编码、参考号码解释、长度哲学以及不同符合类别的命令细节。 ModBus/TCP协议是一个基于TCP/IP的通信协议，用于PLC（可编程逻辑控制器）和其他设备之间的通信。协议的核心特点包括： 1. **连接导向性**：ModBus/TCP是建立在TCP连接之上的，这意味着在进行数据交换前，必须先建立可靠的TCP连接，提高了数据传输的可靠性。 2. **数据编码**：协议规定了如何编码和解码数据，确保不同设备间的数据交换能够准确无误。这通常涉及到二进制和十六进制的转换，以及错误校验机制如CRC（循环冗余校验）。 3. **参考号码解释**：协议中涉及的寄存器地址或其他参考号码有着特定的含义和用途，例如输入寄存器、输出寄存器、离散输入等。 4. **隐含长度哲学**：在ModBus/TCP中，某些消息的长度可以通过协议规则推断出来，而不是直接在消息中指定，简化了通信过程。文档详细介绍了三种符合类别（Class 0、Class 1、Class 2）的命令： - **Class 0** 包括基础的读写功能，如读取多个寄存器（FC3）、写入多个寄存器（FC16）。 - **Class 1** 包含读取离散输入（FC1）、读取输入寄存器（FC4）、写入单个线圈（FC5）和写入单个寄存器（FC6）等。 - **Class 2** 提供更复杂的功能，如强制多个线圈（FC15）、读取通用引用（FC20）、写入通用引用（FC21）、掩码写寄存器（FC22）、读写寄存器（FC23）等。每个命令都详细描述了其功能、请求格式和响应格式，帮助开发者理解和实现ModBus/TCP协议。综上，ModBus/TCP v1.0是一个为工业自动化设备提供标准化通信的协议，通过TCP/IP网络连接实现设备间的通讯，支持多种操作，如读取和写入数据，以及控制设备状态等。这份规范对于理解和实现基于ModBus/TCP的系统至关重要。

openmbus.doc 5 3/29/99

numbers were expressed as decimal numbers with a starting offset of 1. However MODBUS uses the more

natural software interpretation of an unsigned integer index starting at zero.

So a modbus message requesting the read of a register at offset 0 would return the value known to the

application programmer as found in register 4:00001 (memory type 4 = output register, reference 00001)

2.4 Implied length philosophy

All MODBUS requests and responses are designed in such a way that the recipient can verify that a

message is complete. For function codes where the request and response are of fixed length, the function

code alone is sufficient. For function codes carrying a variable amount of data in the request or response,

the data portion will be preceded by a byte count.

When Modbus is carried over TCP, additional length information is carried in the prefix to allow the recipient

to recognize message boundaries even if the message had to be split into multiple packets for transmission.

The existence of explicit and implicit length rules, and use of a CRC-32 error check code (on Ethernet) results

in an infinitesimal chance of undetected corruption to a request or response message.

3. Conformance class summary

When defining a new protocol from scratch, it is possible to enforce consistency of numbering and

interpretation. MODBUS by its nature is implemented already in many places, and disruption to existing

implementations must be avoided.

Therefore the existing set of transaction types have been classified into conformance classes where level 0

represents functions which are universally implemented and totally consistent, and level 2 represents useful

functions but with some idiosyncrasies. Those functions of the present set which are NOT suitable for

interoperability are also identified.

It must be noted that future extensions to this standard may define additional function codes to handle

situations where the existing de-facto standard is deficient. However, it would be misleading for details of

such proposed extensions to appear in this document. It will always be possible to determine if a particular

target device supports a particular function code by sending it ‘speculatively’ and checking for the type of

exception response if any, and this approach will guarantee the continued interoperability of current

MODBUS devices with the introduction of any such extensions. Indeed, this is the philosophy which has

led to the current function code classification.

3.1 Class 0

This is the minimum useful set of functions, for both a MASTER and a SLAVE.

read multiple registers (fc 3)

write multiple registers (fc 16)

3.2 Class 1

openmbus.doc 6 3/29/99

This is the additional set of functions which is commonly implemented and interoperable. As explained

before, many slaves choose to treat input, output, discrete and register as equivalent.

read coils (fc 1)

read input discretes (fc 2)

read input registers (fc 4)

write coil (fc 5)

write single register (fc 6)

read exception status (fc 7)

This function typically has a different meaning for each slave family

3.3 Class 2

These are the data transfer functions needed for routine operations such as HMI and supervision

force multiple coils (fc 15)

read general reference (fc 20)

This function has the ability to handle multiple simultaneous requests, and can accept a

reference number of 32 bits. Current 584 and 984 PLC’s only use this function to accept

references of type 6 (extended register files).

This function would be the most appropriate to extend to handle large register spaces and

data items which currently lack reference numbers such as ‘unlocated’ variables.

write general reference (fc 21)

This function has the ability to handle multiple simultaneous requests, and can accept a

reference number of 32 bits. Current 584 and 984 PLC’s only use this function to accept

references of type 6 (extended register files).

This function would be the most appropriate to extend to handle large register spaces and

data items which currently lack reference numbers such as ‘unlocated’ variables.

mask write register (fc 22)

read/write registers (fc 23)

This function allows the input of a range of registers and the output of a range of registers

as a single transaction. It is the most efficient way, using MODBUS, to perform a regular

exchange of a state image such as with an I/O module.

Thus a high performance but versatile data collection device might choose to implement

functions 3, 16 and 23 to combine rapid regular exchange of data (23) with the ability to

perform on-demand interrogations or updates of particular data items (3 and 16)

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