Introduction to Industrial Automation

8.2   Communication  Protocols

The term “protocol” in the field of communication networks generally refers to the rules according 

which a node can have access to the network bus. Access rules essentially constitute the control 

methods for the information flow in the network, which ensure that there will not be collisions 

PLC

PLC

PLC

PLC

PLC

Figure 8.4  Bus topology of a PLC network.

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  Introduction to Industrial Automation

in the channel of communication that could lead the network to instability, and that they will 

not interrupt or corrupt small or large amounts of transmitted information. The control of nodes’ 

access may be central or distributed.

 

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In the case of centralized control, the access of PLCs to the network is determined by a cen-

tral PLC called a “master PLC”.

 

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In the case of distributed control, all PLCs of the network have the same rights to send and 

receive messages. Therefore, they don’t have to wait in order to receive rights from a master 

PLC.

Access methods of the first category are deterministic, because each PLC communicates 

with another in a given time period. Access methods of the second category may be determinis-

tic or stochastic. In the case of stochastic methods, the time a PLC communicates with another 

PLC is not definite, and is generally dependent on different access methods that also define the 

access times.