Introduction to Industrial Automation

Figure 8.7  Token bus network with a logical ring.

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Industrial Networks of PLCs 

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coaxial cables, the 50 Ω cable used for digital transmission (base band) and the 75 Ω cable 

used for analog transmission (wide band). The coaxial cable is characterized by a high band-

width and a satisfactory tolerance to noise. The bandwidth varies depending on the length of 

the cable. For coaxial cables with a length of 1 km, the data rate can be as high as 10 Mbps. 

For smaller or larger distances the data rate is correspondingly higher or lower. Coaxial cables 

are used by many companies in commercial industrial networks for the interconnection of 

PLCs.

The 75 Ω coaxial cable through which analog signal transmission is performed, is called a 

broadband cable, and is used between others in cable television systems for transmitting video, 

voice, and data. It is characterized by a high bandwidth and a long transmission distance. In 

wideband transmission systems, the total available bandwidth is divided into a series of non-

overlapping frequency channels, usually in 6 MHz channels, used for television broadcast. 

Each channel could transmit analog TV signals, audible signals, and digital data indepen-

dently from other channels. Video channels, after being digitized, could be compressed to 

better fit the channel width. For transmitting digital signals in an analog network, every 

digital connected device should be equipped with electronic circuits able to convert the out-

bound train of bits into an analog signal and the incoming analog signal into a train of bits. 

In conclusion, the 75 Ω coaxial cable is suitable for all the types of digital audio, digital video, 

and data signals.

Fiber-optic cable. Digital data transmission via an optical fiber can now be considered as a 

classic broadband trend. Many manufacturers of PLCs have adopted optical fibers as the transmis-

sion medium in the respective networks on the market. It is known that a light pulse can be used 

to represent the binary digit “1”, while the absence of a pulse can represent the binary digit “0”. 

To perform an optical transmission, a light source, a transmission medium, and a light detector 

are required. The transmission medium is a very thin filament of glass or fused silica. The light 

source is either an LED or a laser diode that both emit pulses of light when voltage is applied. The 

detector is a photodiode that produces an electrical signal in response to an incident of light. By 

connecting an LED or a laser diode at one end of a fiber optic cable and a photodiode at the other 

end, a one-way data transmission system can be created, which is able to accept an electrical signal 

to convert and transmit light pulses subsequently, and then, at the other end or at the signal exit, 

reconvert it into a similar electrical signal as the input one.