Introduction to Industrial Automation

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Chapter 8

Industrial Networks of PLCs

PLCs, which are the leading, stand-alone control tools of industrial processes today, can be inter-

connected with each other to exchange data, information, and control signals. The current form 

of automation of most modernized industries is based on integrated control systems implemented 

by the communicative interconnection of computers and PLCs. The interconnected systems of 

PLCs offer to an extensive industrial process, flexibility, transparency, and effective coordination 

of all its parts. The utilization of PLC networks has a direct huge economic benefit, as has been 

proven in practice in terms of quality improvement, reduction of production costs, flexibility, and 

reliability of industrial production.

It is an undisputable fact that it is impossible for an industrial process to take place in a limited 

space or in a single building. Typically, an entire industrial process is implemented in a number 

of buildings (production departments) and therefore a need is created for effective coordination 

of these various sections of production. Initially, during the first decades of the industrial age, 

a lot of time was wasted and labor costs were high mainly due to the need to send “messages” 

along all sub-processes in order to achieve better process operation and exploitation. The first step 

towards automation is the introduction of central control stations for the production process. 

However, the management and control of an industrial process is effective only if information 

can be received from every point in the network. This means that for every “bit” of information 

sent to the central control station, a corresponding wire is needed. Therefore, for an increased 

volume of transmitted information, an increased amount of wires or, equivalently, a significant 

cabling task is needed. Hence, it is understandable that in order to install a central station for 

monitoring and controlling an industrial process, many, large bundles of wires are needed (peer-

to-peer connections), which leads to a significant increase in the cost of the automation task. At 

this time, the idea of using communication networks in the industrial world has matured as a 

physical step after the parallel developments in the area of computer networks. With the help of 

communication networks, all the bundles of cables are replaced by a single cable, which contains 

only two or four conductors instead of the thousands that we had before. In this approach, digital 

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  Introduction to Industrial Automation

devices of any kind, participating in the operation of an industrial process, are connected via a 

network, and can communicate and exchange data with each other and with a central monitor-

ing station. The data received by a digital device can relate the information required for its own 

control decisions, or the automation commands of a coordinator PLC for specific control actions 

directly enforceable to the actuators, etc. In conclusion, it can be said that one of the purposes of 

using communication networks in an industrial environment is to reduce the enormous task of 

wiring. The essential result is a significant reduction in installation, pipeline, and cable channel 

costs, as well as maintenance costs of wiring. But the financial benefit is not the only profit that 

can be achieved from the application of computer networks in industrial automations. With the 

use of communication networks in an industry, we have the ability to receive and process any 

data desired in a central control station. This, in addition to many others, leads to faster and more 

accurate production planning, as well as faster optimization and tuning of controlled processes 

based on reference values received from the central control station. With the use of communica-

tion networks, increased operational reliability is also achieved. In the networked automation 

approach, errors are detected at an early stage, while the source of errors can be easily identified 

using the existing data of a control process. With the adoption of communication networks, 

individual processes can be controled independently and take place in parallel. This has, as a 

consequence, the whole industrial process to be controlled in a decentralized manner and at 

high speed from a central control station providing coordination, monitoring, and recording of 

data. The communication networks developed for the use in an industrial environment are called 

“industrial networks”. They interconnect primarily programmable logic controllers, as shown 

in Figure 8.1, and secondly personal computers or any other form of digital communication -

enabled devices, such as a robot controller.