ETHERNET • 269
Important terms
Modular I/O System
ETHERNET TCP/IP
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 trans-
mitted as a result of the large amount of data traffic. The delay time in a
Shared ETHERNET cannot be easily calculated or predicted.
Fig. 5-6: Principle of Shared ETHERNET
G012910e
Deterministic ETHERNET
The TCP/IP software or the user program in each subscriber can limit trans-
mittable messages to make it possible to determine real-time requirements. At
the same time the maximum medium message rate (datagram per second), the
maximum medium duration of a message, and the minimum time interval
between the messages (waiting time of the subscriber) is limited.
Therefore, the delay time of a message is predictable.
Switched ETHERNET
In the case of Switched Ethernet, several fieldbus nodes are connected by a
switch. When data from a network segment reaches the switch, it saves the
data and checks for the segment and the node to which this data is to be sent.
The message is then only sent to the node with the correct target address.
This reduces the data traffic over the network, extends the bandwidth and
prevents collisions. The runtimes can be defined and calculated, making the
Switched Ethernet deterministic.
Fig. 5-7: Principle of Switched ETHERNET
G012909e
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