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Modbus是一种广泛应用于工业自动化领域的通信协议，它允许设备之间进行简单、有效的数据交换。这份文档《Modbus Application Protocol V1.1b》是关于Modbus应用层协议的详细规格说明，版本为V1.1b，对于理解和实现Modbus通信至关重要。 Modbus是由Modicon公司（现为施耐德电气的一部分）于1979年开发的，它是一种基于串行通信的主从协议。协议的基础是ASCII或RTU（Remote Terminal Unit）传输模式，用于在不同类型的电子设备间建立通信，如PLC（可编程逻辑控制器）、HMI（人机界面）、SCADA（监控与数据采集系统）等。 **Modbus协议结构：** 1. **功能码**：每个Modbus请求或响应都包含一个功能码，指示要执行的操作，如读寄存器、写寄存器、读线圈状态等。功能码范围通常从0x01到0xFF，其中某些码已被保留或未定义。 2. **数据域**：根据功能码，数据域包含必要的参数和/或响应数据，例如寄存器地址、数量、线圈状态等。 3. **起始地址和数据点数**：在读写操作中，这些字段指定了要访问的设备内存位置及数量。 4. **校验和**：为了确保数据传输的准确性，协议通常包括CRC（循环冗余校验）或LRC（纵向冗余校验）。 **Modbus协议类型：** 1. **Modbus RTU (Remote Terminal Unit)**：最常用的Modbus变体，使用二进制格式，效率高但需要同步。 2. **Modbus ASCII (American Standard Code for Information Interchange)**：使用ASCII字符编码，错误检测能力更强，但传输速度较慢。 3. **Modbus TCP/IP**：将Modbus协议与TCP/IP网络协议栈结合，适应以太网和互联网环境。 **Modbus通信模型：** 1. **主设备（Master）**：发起通信请求的设备，可以向任何从设备发送请求。 2. **从设备（Slave）**：接收并响应主设备请求的设备，每个从设备都有一个唯一的地址（0到247）。 **应用实例：** 1. **读取线圈状态**：主设备可以请求从设备的某个线圈（逻辑状态）的当前值。 2. **读取输入寄存器**：获取从设备的输入寄存器（无源，只读）的数据。 3. **读取保持寄存器**：读取从设备的保持寄存器（可读写）的数据。 4. **写单个线圈**：设置从设备的某个线圈状态。 5. **写单个寄存器**：修改从设备的某个保持寄存器的值。 6. **写多个寄存器**：批量写入多个保持寄存器的值。《Modbus Application Protocol V1.1b》文档会深入解释这些概念，并提供标准操作的详细规格，包括错误处理、异常响应以及具体的数据格式和编码规则。通过阅读这份文档，开发者可以更好地理解如何在实际项目中实施和调试Modbus通信，确保不同设备间的无缝交互。

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Modbus-IDA

December 28, 2006 http://www.Modbus-IDA.org 1/51

MODBUS APPLICATION PROTOCOL SPECIFICATION

V1.1b

CONTENTS

Introduction ................................................................................................................... 2

1.1 Scope of this document ........................................................................................ 2

2 Abbreviations ................................................................................................................ 2

3 Context ......................................................................................................................... 3

4 General description ....................................................................................................... 3

4.1 Protocol description.............................................................................................. 3

4.2 Data Encoding...................................................................................................... 6

4.3 MODBUS Data model ........................................................................................... 6

4.4 MODBUS Addressing model ................................................................................. 7

4.5 Define MODBUS Transaction................................................................................ 8

5 Function Code Categories ............................................................................................10

5.1 Public Function Code Definition ...........................................................................11

6 Function codes descriptions .........................................................................................12

6.1 01 (0x01) Read Coils...........................................................................................12

6.2 02 (0x02) Read Discrete Inputs............................................................................13

6.3 03 (0x03) Read Holding Registers .......................................................................15

6.4 04 (0x04) Read Input Registers ...........................................................................16

6.5 05 (0x05) Write Single Coil ..................................................................................17

6.6 06 (0x06) Write Single Register ...........................................................................19

6.7 07 (0x07) Read Exception Status (Serial Line only)..............................................20

6.8 08 (0x08) Diagnostics (Serial Line only)...............................................................21

6.8.1 Sub-function codes supported by the serial line devices ...........................22

6.8.2 Example and state diagram......................................................................24

6.9 11 (0x0B) Get Comm Event Counter (Serial Line only) .........................................25

6.10 12 (0x0C) Get Comm Event Log (Serial Line only) ...............................................26

6.11 15 (0x0F) Write Multiple Coils..............................................................................29

6.12 16 (0x10) Write Multiple registers ........................................................................30

6.13 17 (0x11) Report Slave ID (Serial Line only) ........................................................32

6.14 20 (0x14) Read File Record .................................................................................32

6.15 21 (0x15) Write File Record .................................................................................34

6.16 22 (0x16) Mask Write Register.............................................................................36

6.17 23 (0x17) Read/Write Multiple registers ...............................................................38

6.18 24 (0x18) Read FIFO Queue................................................................................41

6.19 43 ( 0x2B) Encapsulated Interface Transport .......................................................42

6.20 43 / 13 (0x2B / 0x0D) CANopen General Reference Request and Response

PDU ....................................................................................................................

6.21 43 / 14 (0x2B / 0x0E) Read Device Identification..................................................44

7 MODBUS Exception Responses ...................................................................................48

Annex A (Informative): MODBUS RESERVED FUNCTION CODES, SUBCODES AND

MEI TYPES ..................................................................................................................

Annex B (Informative): CANOPEN GENERAL REFERENCE COMMAND .............................51

MODBUS Application Protocol Specification V1.1b Modbus-IDA

December 28, 2006 http://www.Modbus-IDA.org 2/51

1 Introduction

1.1 Scope of this document

MODBUS is an application layer messaging protocol, positioned at level 7 of the OSI model,

that provides client/server communication between devices connected on different types of

buses or networks.

The industry’s serial de facto standard since 1979, MODBUS continues to enable millions of

automation devices to communicate. Today, support for the simple and elegant structure of

MODBUS continues to grow. The Internet community can access MODBUS at a reserved

system port 502 on the TCP/IP stack.

MODBUS is a request/reply protocol and offers services specified by function codes.

MODBUS function codes are elements of MODBUS request/reply PDUs. The objective of this

document is to describe the function codes used within the framework of MODBUS

transactions.

MODBUS is an application layer messaging protocol for client/server communication between

devices connected on different types of buses or networks.

It is currently implemented using:

y TCP/IP over Ethernet. See MODBUS Messaging Implementation Guide V1.0a.

y Asynchronous serial transmission over a variety of media (wire : EIA/TIA-232-E, EIA-

422, EIA/TIA-485-A; fiber, radio, etc.)

y MODBUS PLUS, a high speed token passing network.

TCP

Modbus on TCP

MODBUS APPLICATION LAYER

Ethernet

Physical layer

Ethernet II /802.3

EIA/TIA-232 or

EIA/TIA-485

Master / Slave

Physical layer

MODBUS+ / HDLC

Other

Figure 1: MODBUS communication stack

References

1. RFC 791, Internet Protocol, Sep81 DARPA

2 Abbreviations

ADU Application Data Unit

HDLC High level Data Link Control

HMI Human Machine Interface

IETF Internet Engineering Task Force

I/O Input/Output

MODBUS Application Protocol Specification V1.1b Modbus-IDA

December 28, 2006 http://www.Modbus-IDA.org 3/51

IP Internet Protocol

MAC Medium Access Control

MB MODBUS Protocol

MBAP MODBUS Application Protocol

PDU Protocol Data Unit

PLC Programmable Logic Controller

TCP Transport Control Protocol

3 Context

The MODBUS protocol allows an easy communication within all types of network

architectures.

PLC PLCHMI I/ O I/ O

I/ O

Drive

MODBUS ON TCP/IP

Gateway

Device

HMI

PLC

Drive

I/ O

Device

MODBUS COMMUNICATION

Figure 2: Example of MODBUS Network Architecture

Every type of devices (PLC, HMI, Control Panel, Driver, Motion control, I/O Device…) can use

MODBUS protocol to initiate a remote operation.

The same communication can be done as well on serial line as on an Ethernet TCP/IP

networks. Gateways allow a communication between several types of buses or network using

the MODBUS protocol.

4 General description

4.1 Protocol description

The MODBUS protocol defines a simple protocol data unit (PDU) independent of the

underlying communication layers. The mapping of MODBUS protocol on specific buses or

network can introduce some additional fields on the application data unit (ADU).

MODBUS Application Protocol Specification V1.1b Modbus-IDA

December 28, 2006 http://www.Modbus-IDA.org 4/51

dditional address Function code Data Error check

PDU

Figure 3: General MODBUS frame

The MODBUS application data unit is built by the client that initiates a MODBUS transaction.

The function indicates to the server what kind of action to perform. The MODBUS application

protocol establishes the format of a request initiated by a client.

The function code field of a MODBUS data unit is coded in one byte. Valid codes are in the

range of 1 ... 255 decimal (the range 128 – 255 is reserved and used for exception

responses). When a message is sent from a Client to a Server device the function code field

tells the server what kind of action to perform. Function code "0" is not valid.

Sub-function codes are added to some function codes to define multiple actions.

The data field of messages sent from a client to server devices contains additional

information that the server uses to take the action defined by the function code. This can

include items like discrete and register addresses, the quantity of items to be handled, and

the count of actual data bytes in the field.

The data field may be nonexistent (of zero length) in certain kinds of requests, in this case

the server does not require any additional information. The function code alone specifies the

action.

If no error occurs related to the MODBUS function requested in a properly received MODBUS

ADU the data field of a response from a server to a client contains the data requested. If an

error related to the MODBUS function requested occurs, the field contains an exception code

that the server application can use to determine the next action to be taken.

For example a client can read the ON / OFF states of a group of discrete outputs or inputs or

it can read/write the data contents of a group of registers.

When the server responds to the client, it uses the function code field to indicate either a

normal (error-free) response or that some kind of error occurred (called an exception

response). For a normal response, the server simply echoes to the request the original

function code.

Function code

Data Request

Client Serve

Initiate request

Perform the action

Initiate the response

Receive the response

Function code Data Response

Figure 4: MODBUS transaction (error free)

For an exception response, the server returns a code that is equivalent to the original

function code from the request PDU with its most significant bit set to logic 1.

MODBUS Application Protocol Specification V1.1b Modbus-IDA

December 28, 2006 http://www.Modbus-IDA.org 5/51

Client Server

Initiate request

Error detected in the action

Initiate an error

Exception Function code

Receive the response

Exception code

Function code Data Request

Figure 5: MODBUS transaction (exception response)

) Note: It is desirable to manage a time out in order not to indefinitely wait for an answer which will perhaps

never arrive.

The size of the MODBUS PDU is limited by the size constraint inherited from the first

MODBUS implementation on Serial Line network (max. RS485 ADU = 256 bytes).

Therefore:

MODBUS PDU for serial line communication = 256 - Server address (1 byte) - CRC (2

bytes) = 253 bytes.

Consequently:

RS232 / RS485 ADU = 253 bytes + Server address (1 byte) + CRC (2 bytes) = 256 bytes.

TCP MODBUS ADU = 253 bytes + MBAP (7 bytes) = 260 bytes.

The MODBUS protocol defines three PDUs. They are :

• MODBUS Request PDU, mb_req_pdu

• MODBUS Response PDU, mb_rsp_pdu

• MODBUS Exception Response PDU, mb_excep_rsp_pdu

The mb_req_pdu is defined as:

mb_req_pdu = {function_code, request_data}, where

function_code = [1 byte] MODBUS function code,

request_data = [n bytes] This field is function code dependent and usually

contains information such as variable references,

variable counts, data offsets, sub-function codes etc.

The mb_rsp_pdu is defined as:

mb_rsp_pdu = {function_code, response_data}, where

function_code = [1 byte] MODBUS function code

response_data = [n bytes] This field is function code dependent and usually

contains information such as variable references,

variable counts, data offsets, sub-function codes, etc.

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