Modbus TCP/IP协议实现指南

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"Modbus Messaging on TCP/IP Implementation Guide V1.0b是Modbus Organization发布的一份关于在TCP/IP上实现Modbus通信的详细指南，旨在帮助开发者理解和应用Modbus协议。该文档涵盖了Modbus通信的基本原理、客户端/服务器模型、相关文档、缩略语、上下文、协议描述、功能描述以及实现指南等多个方面。" 本文档首先介绍了Modbus协议的基础知识，包括其作为单主站的主/从通信模式，强调在网络中只能有一个主站，而从站的地址范围是1-247，其中0是广播地址。此外，Modbus支持多种传输介质，如RS232C、RS485和TCP/IP等，并提供了ASCII和RTU两种串行传输模式，RTU通常更常用。接着，文档详细阐述了Modbus在TCP/IP环境下的应用数据单元（ADU）结构，以及MODBUS应用协议（MBAP）头的组成，包括事务标识符、协议标识符、单元标识符和长度字段。文档还详细列出了Modbus的各种功能代码，这些代码用于定义不同的Modbus操作，如读寄存器、写寄存器等。在功能描述部分，文档讨论了Modbus组件架构模型，TCP连接管理，包括连接管理模块、操作模式对TCP连接的影响以及访问控制模块。此外，它还详细解释了如何利用TCP/IP堆栈，如使用BSD套接字接口，TCP和IP层的参数化，以及通信应用层的设计，包括Modbus客户端和服务器的角色和行为。最后，文档提供了一份实现指南，包含对象模型图，指导开发者如何构建TCP管理和配置层包，以确保有效、安全地实现Modbus通信。 "Modbus Messaging on TCP/IP Implementation Guide V1.0b"是理解、开发和集成Modbus TCP/IP通信系统的关键参考资料，对于那些需要在工业自动化或物联网环境中使用Modbus协议的工程师来说尤其有价值。

MODBUS Messaging on TCP/IP Implementation Guide V1.0b

Primary tables Object type Type of Comments

Discretes Input Single bit Read-Only

This type of data can be provided by an I/O system.

Coils Single bit Read-Write

This type of data can be alterable by an application

program.

Input Registers 16-bit word Read-Only

This type of data can be provided by an I/O system

Holding Registers 16-bit word Read-Write

This type of data can be alterable by an application

program.

Input Discrete

MODBUS access

Device application memory

MODBUS SERVER DEVICE

MODBUS Request

Coils

Input Registers

Output Registers

Figure 5 MODBUS Data Model with

separate blocks

Device application memory

MODBUS SERVER DEVICE

MODBUS Request

Input Discrete

MODBUS access

Coils

Input Registers

Output Registers

Figure 6 MODBUS Data Model with only

1 block

¾ MODBUS Client

The MODBUS Client allows the user application to explicitly control information

exchange with a remote device. The

MODBUS Client builds a MODBUS request from

parameter contained in a demand sent by the user application to the MODBUS Client

Interface.

The MODBUS Client uses a MODBUS transaction whose management includes waiting

for and processing of a MODBUS confirmation.

¾ MODBUS Client Interface

The MODBUS Client Interface provides an interface enabling the user application to

build the requests for various MODBUS

services including access to MODBUS

application objects. The MODBUS Client interface (API) is not part of this

Specification, although an example is described in the implementation model.

¾ MODBUS Server

On reception of a MODBUS request this module activates a local action to read, to

write

or to achieve some other actions. The processing of these actions is done totally

transparently for the application programmer. The main MODBUS server functions are

to wait for a MODBUS request on 502 TCP port, to treat this request and then to build a

MODBUS response depending on device context.

¾ MODBUS Backend Interface

The MODBUS Backend Interface is an interface from the MODBUS Server to

the user

application in which the application objects are defined.

October 24, 2006

http://www.Modbus.org

8/46

Informative Note: The Backend Interface is not defined in this Specification

Modbus Organization

MODBUS Messaging on TCP/IP Implementation Guide V1.0b

October 24, 2006

http://www.Modbus.org

9/46

• TCP Management layer

Informative Note: The TCP/IP discussion in this Specification is based in part upon

reference

[2] RFC 1122 to assist the user in implementing the MODBUS Application

Protocol Specification [1] over TCP/IP.

One of the main functions of the messaging service is to manage communication

establishment and ending and to manage the data flow on established

TCP

connections.

¾ Connection Management

A communication between a client and server MODBUS

Module requires the use of a

TCP connection management module. It is in charge to manage globally messaging

TCP connections.

Two possibilities are proposed for the connection management. Either the user

application itself manages TCP connections or the connection management is totally

done by this module and therefore it is transparent for the user application. The last

solution implies less flexibility.

The listening TCP port 502 is reserved for MODBUS communications. It is

mandatory to listen by default on that port. However, some markets or applications

might require that another port is dedicated to MODBUS over TCP. For that reason, it

is highly recommended that the clients and the servers give the possibility to the user

to parameterize the MODBUS over TCP port number. It is important to note that

even if another TCP server port is configured for MODBUS service in certain

applications, TCP server port 502 must still be available in addition to any

application specific ports.

¾ Access Control Module

In certain critical contexts, accessibility to internal

data of devices must be forbidden for

undesirable hosts. That’s why a security mode is needed and security process may be

implemented if required.

• TCP/IP Stack layer

The TCP/IP stack can be parameterized in order to adapt the data flow control, the

address management and the connection management to different constraints specific

to a product or to a system. Generally the BSD

socket interface is used to manage the

TCP connections.

 Resource management and Data flow control

In order to equilibrate inbound and outbound messaging data flow between the

MODBUS client and the server, data flow control mechanism is provided in all layers

of MODBUS messaging stack.

The resource management and flow control module is first based on TCP internal flow

control added with some data flow control in the data link

layer and also in the user

application level.

Modbus Organization

MODBUS Messaging on TCP/IP Implementation Guide V1.0b

October 24, 2006

http://www.Modbus.org

10/46

4.2 TCP CONNECTION MANAGEMENT

4.2.1 Connections management Module

4.2.1.1 General description

A MODBUS communication requires the establishment of a TCP connection between a

Client and a Server.

The establishment of the connection can be activated either explicitly by the User

Application module or automatically by the TCP connection management module.

In the first case an application-programming interface has to be provided in the user

application module to manage completely the

connection.

This solution provides

flexibility for the application programmer but it requires a good expertise on TCP/IP

mechanism.

In the second case the TCP connection management is completely hidden to the user

application that only sends and receives MODBUS messages. The TCP connection

management module is in charge to establish a new TCP connection when it is

required.

The definition of the number of TCP client and server connections is not on the scope of

this document (value n in this document). Depending on the device

capacities the

number of TCP connections can be different.

Implementation Rules :

1) Without explicit user requirement, it is recommended to implement the automatic TCP

connection management

2) It is recommended to keep the TCP connection opened with a remote device and not

to open and close it for each MODBUS/TCP transaction,

Remark: However the MODBUS client must be capable of accepting a close request

from

the server and closing the connection. The connection can be reopened when

required.

3) It is recommended for a MODBUS Client to open a minimum of TCP connections with

a remote MODBUS server (with the same IP address). One connection per application

could be a good choice.

4) Several MODBUS transactions can be activated simultaneously on the same TCP

Connection.

Remark: If this is done then the MODBUS transaction identifier must be used to

uniquely identify the matching requests and responses.

5) In case of a bi-directional communication between two remote MODBUS entities (

each of them is client and server), it is necessary to open separate connections for

the client data flow and for the server data flow.

6) A TCP frame must transport only one MODBUS ADU. It is advised against sending

multiple MODBUS requests or responses on the same TCP PDU

Modbus Organization

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