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TCP segment

The packet header of a TCP data packet is comprised of at least 20 bytes and contains, among others, the application port number of the transmitter and the receiver, the sequence number and the acknowledgement number.

The resulting TCP packet is used in the data unit area of an IP packet to create a TCP/IP packet.

5.3.3.3 UDP

The UDP protocol, like the TCP protocol, is responsible for the transport of data. Unlike the TCP protocol, UDP is not connection-orientated; meaning that there are no control mechanisms for the data exchange between transmitter and receiver. The advantage of this protocol is the efficiency of the transmitted data and the resulting higher processing speed.

5.3.3.4 ARP

ARP (Address Resolution Protocol).

This protocol combines the IP address with the physical MAC address of the respective Ethernet card. It is always used when data transfer to an IP address takes place in the same logical network in which the sender is located.

5.3.4 Application Protocols

In addition to the communication protocols described above, various application protocols are implemented in the WAGO ETHERNET Coupler. These protocols allow the user easy access to the fieldbus nodes:

•MODBUS TCP (UDP)

•Ethernet/IP

Further protocols serve for the administration and diagnosis of the system:

•BootP

•HTTP

•DHCP

•DNS

•SNTP

•FTP

•SNMP

•SMTP.

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5.3.4.1 MODBUS

MODBUS is a manufacturer-independent, open fieldbus standard for diverse applications in manufacturing and process automation.

The MODBUS protocol is implemented for the transmission of the process image, the fieldbus variables, different settings and information on the controller according to the current Internet Draft.

The data transmission in the fieldside takes place via TCP and via UDP.

The MODBUS/TCP protocol is a variation of the MODBUS protocol, which was optimized for communication via TCP/IP connections.

This protocol was designed for data exchange in the field level (i.e. for the exchange of I/O data in the process image).

All data packets are sent via a TCP connection with the port number 502.

For the MODBUS protocol five connections are made available over TCP. Thus it allows digital and analog output data to be directly read out at a fieldbus node and special functions to be executed by way of simple MODBUS function codes from five stations simultaneously.

For this purpose a set of MODBUS functions from the OPEN MODBUS /TCP SPECIFICATION is realized.

More information

These functions and their application are described in detail in Chapter 6 "MODBUS functions".

MODBUS TCP segment

The general MODBUS/TCP header is as follows:

Fig. 5-12: MODBUS/TCP Header

More information

The structure of a datagram is specific for the individual function. Refer to Chapter "MODBUS functions".

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5.3.4.2 Ethernet/IP

Ethernet/IP stands for Ethernet Industrial Protocol and defines an open industry standard that extends the classic Ethernet with an industrial protocol. This standard was jointly developed by ControlNet International (CI) and the Open DeviceNet Vendor Association (ODVA) with the help of the Industrial Ethernet Association (IEA).

Ethernet/IP is based on the TCP/IP protocol family and consequently uses the bottom 4 layers of the OSI layer model in unaltered form so that all standard Ethernet communication modules such as PC interface cards, cables, connectors, hubs and switches can also be used with Ethernet/IP.

Positioned above the transport layer is the encapsulation protocol, which enables use of the Control & Information Protocol (CIP) on TCP/IP and UDP/IP.

CIP, as a major network independent standard, is already used with ControlNet and DeviceNet.

Data exchange takes place with the help of an object model.

In this way, ControlNet, DeviceNet and Ethernet/IP have the same application protocol and can therefore jointly use device profiles and object libraries. These objects enable plug-and-play interoperability between complex devices of different manufacturers.

Further information

A detailed description of the object model and its application is available in Chapter 7 "Ethernet/IP (Ethernet/Industrial Protocol).

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5.3.4.3 BootP (Bootstrap Protocol)

The BootP protocol defines a request/response mechanism with which the MAC-ID of a fieldbus node can be assigned a fix IP address.

For this a network node is enabled to send requests into the network and call up the required network information, such as the IP address of a BootP server. The BootP server waits for BootP requests and generates the response from a configuration database.

The dynamic configuration of the IP address via a BootP server offers the user a flexible and simple design of his network. The WAGO BootP server allows any IP address to be easily assigned for the WAGO fieldbus coupler/controller. You can download a free copy of the WAGO BootP server over the Internet at: http://www.wago.com.

More information

The procedure for address allocation with the WAGO BootP Server is described in detail in the Chapter “Starting up an ETHERNET TCP/IP fieldbus node”.

The BOOTP Client allows for dynamic configuring of the network parameters:

When using the bootstrap protocol for configuring the node, the network parameters (IP address, etc... ) are stored in the EEPROM.

Note

The network configuration is only stored in the EEPROM when the BootP protocol is used, although not if configuration is done via DHCP.

The BootP protocol is activated in the controller by default.

When the BootP protocol is activated, the controller expects a BootrP server to be permanently present.

If, however, there is no BootP server available after a power-on reset, the network remains inactive.

To operate the controller with the IP configuration stored in the EEPROM, you must first deactivate the BootP protocol.

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