Introduction to Industrial Automation

8.1.3   Bus  Topology

In networks with a bus topology, all PLCs are connected to a common communication line that 

is a multi-point or multi-drop physical medium with open ends for transmitting data-messages, as 

shown in Figure 8.4. This common line is called a “bus” and is the only channel of communica-

tion between the PLCs of the network. Only one PLC can send a message through the common 

communication channel at any time. The message with the address of the destination node is 

transmitted along the bus until received by the PLC that recognizes its address.

The reliability of bus-type networks is quite satisfactory. The only case that may cease the 

operation of the entire network is when the common communication channel becomes totally 

disabled, something that is very rare. Any possible damage to a PLC has no effect on the function-

ing of the other PLCs, except that they cannot communicate with it. In general, the extension of a 

bus-topology network is excellent and straightforward by the addition of new PLCs. This addition 

is performed very easily at any point of the network, without requiring multiple connections and 

without hindering the operation of the network. Due to the above advantages, bus topology is 

prevalent and widespread in the area of industrial networks.