PLCs and PACs – which one is right for you?

Published:  09 March, 2018

Jonathan Wilkins, marketing director of industrial obsolete parts supplier, EU Automation explores the pros and cons of using traditional programmable logic controllers (PLCs) and newer programmable automation controllers (PACs).

A survey carried out by flavoured syrup supplier, Monin in October, 2017 found that 61% of Brits prefer coffee over tea. Tea and British people are synonymous so, unsurprisingly, this finding has come as a shock to many. The finding is a great example of how something new can overtake the traditional.

Like tea and coffee, both PLCs and PACs are used for the same purpose. They are industrial computers that have been adapted for use in manufacturing environments. Both systems can control automation equipment in factories to a highly reliable degree. PLCs have been used by manufacturing industries for over 40 years. PACs are a newer addition to the automation market and are generally more complex systems than PLCs. However, many industry professionals believe that PAC is just another name for a state-of-the-art PLC because advancements in modern technology have shortened the gap between the two.

The biggest difference between PLCs and PACs is the way they are programmed. PACs are programmed using C and C++, making them more adaptable and efficient. On the other hand, PLCs are programmed using ladder logic — a programming language that uses symbols representing an electrical schematic of relays. This form of programming is sequential, making PLCs largely inflexible. The way the two computers are programmed creates distinct capabilities for each of them.

PLCs are single microprocessor based devices. They are typically programmed to carry out simple execution scans, but they have limited memory and discrete input/output (I/O) capacity. This makes PLCs ideal for controlling simple applications with minimal I/O requirements. PLCs should be used for basic control schemes where complex analogue control and motion control are not necessary. In other words, there is no need to pay a higher price for a PAC if the application does not require a higher level of functionality.

However, PLCs are being developed to have more functions. Modern PLCs have built in networks, which enable them to be used to communicate with other PLCs as well as human machine interfaces (HMIs) and supervisory control and data acquisition (SCADA) systems. A PLC might be enough to fulfill all a plant manager’s automation needs.

PACs are the equivalent of an extra caffeine injection because they normally use two or more processors. In this sense, using a PAC could be likened to using a PC and a PLC together. PACs are also made up of various computer based applications, making them more flexible to program. Unlike PLCs, they can multitask easily by operating in multiple domains such as motion, discrete and process control.

PACs are also used because of their interoperability. They are optimal for large applications but can be scaled down for use on smaller ones. Their modular design makes them useful in simplifying expansion processes, as adding or removing components is easy to execute. Their compatibility with other components also allows PACs to communicate with other PLCs as well as higher level computing systems such as manufacturing execution systems (MES) and enterprise resource planning (ERP) systems.

Programmers are starting to push PACs one step further by integrating third party applications and real-time control. Companies may soon be able to run PC applications, such as Windows, in real-time. This maximises flexibility as well as offering integration of control and information. Using PACs alongside other networks can improve the efficiency of the business as whole.

Just as we might crave an espresso when we wake up but opt for a cup of tea for an afternoon pick-me-up, PACS and PLCs have a time and a place that suits them best. However, that doesn’t mean we can have one but not the other.

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