Introduction to Industrial Automation

8.2.2   Carrier Sense with Collision Detection Method of Access

The full name of this access method is “carrier sense multiple access with collision detection” 

(CSMA/CD protocol). The term “carrier sense” derives from the fact that each PLC can sense 

PLC

PLC

PLC

PLC

Master

Active

station

Slave 1

Slave 2

Slave n

Passive

stations

Figure 8.5  A network with one master and multiple slave PLCs.

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Industrial Networks of PLCs 

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the bus (check its availability for transmitting information), while the term “multiple access” is 

defined from the characteristic that all the PLCs in the network have the privilege to access the 

bus. Furthermore, the term “collision detection” is defined from the protocol’s ability to detect 

communication collisions that may occur, application of methods for avoiding them, and guaran-

tee of the proper operation of the network. According to this access method, when a PLC needs to 

transmit a data packet, first it “hears” the bus, which means that it senses whether the line is idle 

and therefore available to be used. If the bus is busy, the PLC waits until it is released, otherwise 

it immediately transmits the intended information. In the case that two or more PLCs start to 

transmit at the same time in the free bus, then a collision will occur, as shown in Figure 8.6. In 

this situation, after the detection of the collision from the PLCs in the same network, all the col-

lided PLCs stop their transmission, wait for the elapse of a random period of time (standby), and 

after that, repeat the whole process again from the beginning until they successfully achieve the 

transmission of their data packets. In this approach, the main disadvantage of this method is that 

theoretically there is no upper limit to the time that a PLC should wait in order to send a message, 

and thus it is characterized as a stochastic method with the characteristic that this back-off time 

can be increased widely until the collapse of the network. This drawback is the main reason that 

the method is not used when real-time control is needed, which normally happens in industrial 

processes.