Introduction to Industrial Automation

8.1.2   Ring  Topology

In networks with a ring topology, all PLCs are connected in a point-to-point configuration, thus 

forming a continuous ring, as shown in Figure 8.3. Every message transmitted from a PLC sender 

contains the address of the destination PLC and is circulated from node to node in the direction 

PLC

PLC

PLC

PLC

PLC

PLC

Figure 8.2  Star topology of a PLC network.

PLC

PLC

PLC

PLC

PLC

PLC

Figure 8.3  Ring topology of a PLC network.

---

Industrial Networks of PLCs 

◾

 

359

of a ring, until it is received from the destination PLC that recognizes its address and is relayed to 

the PLC sender. The PLC receiver, before relaying the message, can notice that it has been read, 

and therefore when the message is returned to the PLC sender, it understands that the message 

has been received correctly. All the PLCs of this network usually have the same rights to send and 

receive messages, and therefore the control of the information flow in the network is distributed. 

Ring topology networks have a high reliability. In case of a failure of one PLC, the messages are 

driven to the adjacent PLC, and thus the network ring remains unbroken. Obviously, if a PLC 

is disabled, all other PLCs continue to communicate normally. The extension of a ring topology 

network is performed relatively easily, because only the PLCs that are adjacent to the new node 

are affected.