Siemens UL 489 Connection to the BDA Plus via the Ethernet Interface, Definition of Key Terms

Breaker Data Adapter (BDA)

WL Circuit Breaker

Connection to the BDA Plus via the Ethernet Interface

In addition to communication via the serial RS232 channel, the BDA Plus features an Ethernet interface. If the BDA Plus is to be addressed via this interface, it must be integrated in the local Ethernet (LAN). This chapter explains a number of key terms and settings.

Ethernet

Unlike the MODBUS, Ethernet does not function according to a master- slave principle. All the stations have equal priority on the bus, which means that any station can be

the sender and/or receiver.

A sender can only send on the bus if no other station is sending at that point. This is due to the fact that the stations are always "listening in" to find out whether any messages are being sent to them or any

senders are currently active.

If a sender has started sending, it checks that the message it has sent is not corrupt. If the message is not corrupt, the send operation continues. If the sender detects that its data is corrupt, it must abort the send operation because a different sender has already started sending data.

After a random time has elapsed, the sender restarts the send operation. This is known as CSMA/CD and, because it is a “random” access procedure, does not guarantee a response within a certain time frame. This largely depends on the bus load, which means that real-time applications cannot yet be implemented with Ethernet.

Definition of Key Terms

An Intranet system is comprised of several Ethernet lines connected to each other via gateways within a company. The structure of an Intranet system can be just as

diverse as that of the Internet:

it can be restricted to one location or distributed worldwide.

Ethernet/Intranet lines are connected to each other using repeaters, bridges/switches, routers and gateways. These modules work at different levels in the ISO/OSI 7-layer model.

The repeater (or star coupler) only regenerates and strengthens the electrical signal; it does not interpret bits. The bridge (or switch) physically separates the networks and performs fault and load disconnection. Filtering and guidance mechanisms are usually implemented. The router decouples the networks at the logical level (protocol level) by means of the specified addresses. Using routing tables, it knows which messages are to be sent to which address. It continues to work, however, on a protocol-dependent basis. The gateway also enables the router to convert services.

This means that it can act as a security mechanism, such as a firewall, while functioning as a proxy.

A proxy is a program in a gateway that acts as both the server and client. It processes requests, translates them if necessary, and forwards them to the addressees. Proxies are also used to control access (firewall) and forward requests for protocols that are not supported. Intranet users in particular are familiar with the Internet/Intranet page caching function offered by proxies.

The Intranet is connected to the Internet via a company proxy, which can also act as a firewall. If a PC (user) wants to access an area of the Intranet from the Internet, the firewall must be informed of which addresses can be accessed from outside.

Graphic 4-5This diagram illustrates the structure of an Ethernet, how an intranet is integrated, and how this is connected to the Internet.

4/22

WL MODBUS Communication and Electronic Accessories • January 2005