Introduction to Industrial Automation

8.2.3   Token Passing Method of Access

According to this method, a PLC gets the token, which is equivalent to the authorization of a 

data transmission, and transmits the data to the desired PLC. After the end of the transmission, 

the token passes to the next PLC, and the previous procedure is followed. The token is a certain 

number of bits, also called a “coded packet”, which circulates throughout the network. Depending 

on the network’s topology, the token circulation could pass in a ring, a bus approach, or a com-

bination of these access methods. Figure 8.7 shows the application of an access method where 

the token passes initially in a bus topology. In this configuration, the token passes only by some 

selected PLCs in the network with a predetermined a priori sequence, creating a logical ring in this 

way. Each PLC of the logical ring knows which PLC is before and after it. When the logical ring 

is created for the first time, the PLC that initially gets the token transmits the data and, after 

PLC

PLC

PLC

PLC

PLC

PLC

PLC

Collision