"详尽介绍Modbus TCP/IP的技术手册，详细42页，全英文，自行下载"

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Introduction To Modbus TCP/IP

__________________________________________________________________

______________________________________________________________________________________

Acromag, Inc. Tel:248-624-1541 Fax:248-624-9234 Email:sales@acromag.com http://www.acromag.com

The traditional OSI model is presented below, along with the simplified 5-

layer TCP/IP Standard (layers 5 & 6 suppressed). In the OSI model, the

functions of communication are divided into seven (or five) layers, with every

layer handling precisely defined tasks. For example, Layer 1 of this model

is the physical layer and defines the physical transmission characteristics.

Layer 2 is the data link layer and defines the bus access protocol. Layer 7

is the application layer and defines the application functions (this is the layer

that defines how device data is to be interpreted).

OSI 7-LAYER MODEL

TCP/IP

Standard

7 Application Used by software applications to prepare

and interpret data for use by the other six

OSI layers below it. Provides the

application interface to the network.

HTTP, FTP, email SMTP & POP3,

CIP™, SNMP, are all found at this layer.

6 Presentation Representation of data, coding type, and

defines used characters. Performs data

and protocol negotiation and conversion

to ensure that data may be exchanged

between hosts and transportable across

the network. Also performs data

compression and encryption.

5 Session Dialing control and synchronization of

session connection. Responsible for

establishing and managing sessions/

connections between applications & the

network. Windows WinSock socket API

is a common session layer manager.

Application

Layer

4 Transport Sequencing of application data, controls

start/end of transmission, provides error

detection, correction, end-to-end

recovery, and clearing. Provides

software flow control of data between

networks. TCP & UDP are found here.

Transport

Layer

3 Network Controls routing, prioritization, setup,

release of connections, flow control.

Establishes/maintains connections over

a network & provides addressing,

routing, and delivery of packets to hosts.

IP, PPP, IPX, & X.25 are found here.

Internet,

Network, or

Internetwork

Layer

2 Data Link Responsible for ensuring reliable

delivery at the lowest levels, including

data frame, error detection and

correction, sequence control, and flow

control. Ethernet (IEEE 802.2) and MAC

are defined at this level.

1 Physical Defines the electrical, mechanical,

functional, and procedural attributes

used to access and send a binary data

stream over a physical medium (defines

the RJ-45 connector & CAT5 cable of

Ethernet).

Network

Access

Layer

Host-to-

Network

Layer

THE OSI NETWORK

MODEL

The OSI Model represents the

basic network architecture.

Each layer of this model uses

the services provided by the

layer immediately below it.

TCP/IP has no specific

mappings to layers 5 and 6 of

this model and these layers

are often omitted when

referring to the TCP/IP stack.

Introduction To Modbus TCP/IP

___________________________________________________________________

______________________________________________________________________________________

Acromag, Inc. Tel:248-624-1541 Fax:248-624-9234 Email:sales@acromag.com http://www.acromag.com

By the OSI Model, we can infer that in order for two devices to be

interoperable on the same network, they must have the same application-

layer protocol. In the past, many network devices have used their own

proprietary protocols and this has hindered their interoperability. This fact

further drove the need for adoption of open network I/O solutions that would

allow devices from a variety of vendors to seamlessly work together, and

this drive for interoperability is a key reason Modbus TCP/IP was created.

Note that in the TCP/IP Standard Model, Ethernet handles the bottom 2

layers (1 & 2) of the seven layer OSI stack, while TCP/IP handles the next

two layers (3 & 4). The application layer lies above TCP, IP, and Ethernet

and is the layer of information that gives meaning to the transmitted data.

With Acromag 9xxEN-40xx Modbus TCP/IP modules, the application layer

protocol is Modbus. That is, Modbus TCP/IP uses Ethernet media and

TCP/IP to communicate using an application layer with the same register

access method as Modbus RTU. Because many manufacturers happen to

support Modbus RTU and TCP/IP, and since Modbus is also widely

understood and freely distributed, Modbus TCP/IP is also considered an

open standard.

So we see that Modbus TCP/IP is based on the TCP/IP protocol family and

shares the same lower four layers of the OSI model common to all Ethernet

devices. This makes it fully compatible with existing Ethernet hardware,

such as cables, connectors, network interface cards, hubs, and switches.

TCP/IP refers to the Transmission Control Protocol and Internet Protocol

which were first introduced in 1974. TCP/IP is the foundation for the World

Wide Web and forms the transport and network layer protocol of the internet

that commonly links all Ethernet installations world-wide. Simply stated,

TCP/IP allows blocks of binary data to be exchanged between computers.

The primary function of TCP is to ensure that all packets of data are

received correctly, while IP makes sure that messages are correctly

addressed and routed. TCP/IP does not define what the data means or how

the data is to be interpreted, it is merely a transport protocol.

To contrast, Modbus is an application protocol. It defines rules for

organizing and interpreting data and is essentially a messaging structure

that is independent of the underlying physical layer. It is freely available and

accessible to anyone, easy to understand, and widely supported by many

manufacturers.

Modbus TCP/IP uses TCP/IP and Ethernet to carry the data of the Modbus

message structure between devices. That is, Modbus TCP/IP combines a

physical network (Ethernet), with a networking standard (TCP/IP), and a

standard method of representing data (Modbus).

THE OSI NETWORK

MODEL

The TCP/IP Stack

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