Fieldbus
Communication
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99
ETHERNET
WAGO-I/O-SYSTEM 750
ETHERNET TCP/IP
4.1.2.5 Important Terms
Data security
If an internal network (Intranet) is to be connected to the public network
(e.g., the Internet) then data security is an extremely important aspect.
Undesired access can be prevented by a
Firewall
.
Firewalls can be implemented in software or network components. They are
interconnected in a similar way to routers as a switching element between
Intranets and the public network. Firewalls are able to limit or completely
block all access to the other networks, depending on the access direction,
the service used and the authenticity of the network user.
Real-time ability
Transmission above the fieldbus system level generally involves relatively
large data quantities. The permissible delay times may also be relatively
long (0.1...10 seconds).
However, real-time behavior within the fieldbus system level is required for
ETHERNET in industry.
In ETHERNET it is possible to meet the real-time requirements by
restricting the bus traffic (< 10 %), by using a master-slave principle, or also
by implementing a switch instead of a hub.
MODBUS/TCP is a master/slave protocol in which the slaves only respond
to commands from the master. When only one master is used, data traffic
over the network can be controlled and collisions avoided.
Shared ETHERNET
Several nodes linked via a hub share a common medium. When a message is
sent from a station, it is broadcast throughout the entire network and is sent
to each connected node. Only the node with the correct target address
processes the message. Collisions may occur and messages have to be
repeatedly transmitted as a result of the large amount of data traffic. The
delay time in a Shared ETHERNET cannot be easily calculated or predicted.
Fig. 4-6: Principle of Shared ETHERNET
G012910e
