Introduction to Industrial Automation

8.1.1   Star  Topology

In networks with a star topology, the connection between several PLCs is made through a central 

PLC, as shown in Figure 8.2. The central PLC accepts messages from the PLCs’ senders and for-

wards them to the destination PLCs, except if the messages are intended for the receiving PLC. 

The star topology of an industrial network has the advantage that the linking of any two PLCs 

requires two communication lines at most. Thus, the time needed from sending until receiving a 

message is relatively small. On the other hand, the fact that the host PLC participates in the pro-

cedure of dispatching any sent message, can lead to situations of overcrowding and, consequently, 

to delays of the messages’ distribution. The extension of a star network is fairly easy, because the 

only nodes involved in this implementation are the new and central PLCs. To achieve the net-

work’s extension, (a) a new communication line should be installed, (b) the two PLCs should be 

connected in the line and (c) the network routing tables that exist on the remaining PLCs should 

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  Introduction to Industrial Automation

be updated for the addition of the new node. The networks with a star topology have relatively 

low reliability. Any damage to the central PLC will result in the shutdown of the entire network, 

while damage to any other PLC does not have a serious impact on the functioning to the network.