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MODBUS Application Protocol Specification MODBUS.ORG

modbus.org http://www.modbus.org/ 1/45

8May02

Content

1 Introduction .......................................................................... 2

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

1.2 References............................................................................. 2

2 Abbreviations ....................................................................... 3

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

4 General description .............................................................. 5

4.1 Protocol description................................................................ 5

4.2 Data Encoding........................................................................ 7

4.3 MODBUS data model ............................................................. 7

4.4 Define MODBUS Transaction ................................................. 9

5 Function Code Categories .................................................. 11

5.1 Public Function Code Definition.............................................12

6 Function codes descripitons................................................ 12

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

6.2 02 (0x02) Read Discrete Inputs..............................................14

6.3 03 (0x03) Read Holding Registers .........................................17

6.4 04 (0x04) Read Input Registers .............................................18

6.5 05 (0x05) Write Single Coil....................................................20

6.6 06 (0x06) Write Single Register .............................................22

6.7 15 (0x0F) Write Multiple Coils................................................25

6.8 16 (0x10) Write Multiple registers ..........................................28

6.9 20 (0x14) Read File Record ...................................................29

6.10 22 (0x16) Mask Write Register ...........................................34

6.11 23 (0x17) Read/Write Multiple registers..............................35

6.12 43 (0x2B) Read Device Identification..................................38

7 MODBUS Exception Responses......................................... 43

MODBUS Application Protocol Specification MODBUS.ORG

modbus.org http://www.modbus.org/ 5/45

8May02

4 General description

4.1 Protocol description

The MODBUS protocol defined 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).

Additional

address

Function code

Data

Error check

ADU

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 (128 – 255

reserved 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.

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 request, 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 the original function code.

Function code

Data Request

Client

Server

Initiate request

Perform the action

Initiate the response

Receive the response

Function code Data Response

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taxiu

2013-09-16

英文版没找到想要的东西
lovejmm

2015-06-05

看了一下，学习可以，感谢共享
hxz45

2014-04-16

一般般啦。。。有点用
cynric_wu_csdn

2014-06-17

最近做一个关于Modbus的项目，东西收藏下来看看了，多谢分享
Chinaguohu

2014-10-16

值得学习和读读

litong860105

粉丝: 3
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