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PLC or PAC, what's the difference?

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One commonly asked question within industrial control is whether to use a PLC (Programmable Logic Controller) or a PAC (Programmable Automation Controller)? This is usually followed with the next question, what is the difference between the two?

Google the question and here is one answer you will get “there is no specific agreement on the technical difference between a PLC and PAC”

We do know there are differences between the two, although, the statement above suggests this might not be quite clear cut. In fact high end PLC's and PAC's have more in common than ever before. 

PLC’s and PAC’s utilise protocols and networks such as Ethernet, Profibus, ControlNET, DeviceNET, and also integrate with software and databases like, OPC (allows Windows programs to communication with devices), and SQL (language used for managing data within databases).

For sure manufacturers are now adding more functionality within their base PLC’s. Things like positioning control are now included in the main unit, for example the Mitsubishi FX5U offers built-in 4-axis positioning as standard.

Mitsubishi FX5U

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It’s now more difficult to decide between the two as manufacturers are pushing their higher end PLC’s into the PAC arena. So where does a PAC start and PLC stop?

What is the difference?

Let’s start at the beginning;

Programming

PLC’s were programmed in the main using a graphical method called Ladder Logic (there are others). This was derived to allow engineers to migrate easily from relay logic to understanding PLC programming. Ladder Logic is still the most widely used programming language today where PLC’s are concerned.

Basic ladder logic example

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PAC’s were programmed in standard programming language such as; C or C++ which was ideally suited to computer programmers.

In addition PAC's offered tag name databasing (tagging) where descriptive names (tags) can be used to describe the function, such as; Tank 1, Tank 2. These functions are assigned specific I/O’s and memory addresses, from which operator interfaces can link data to OPC clients and SQL databases, thus supporting larger scale systems and providing tighter integration with databases.

Functional

PLC’s were originally designed in the late 1960’S to remove the large costs associated in replacing relays within relay controlled applications. Since those early days they have come a long way. 

Today PLC’s are in constant development, there is huge demand for more and more functionality. Some of this functionality is incorporated within the PLC itself such as; increases in control loops, memory and communication ports. Expansion modules also offer increased I/O and functionality, such as; motion control, process control, networking and data manipulation and transfer.

PAC’s are ideal when there is the need for communicating, monitoring and control across various systems and devices. For example the Schneider Electric Modicon M580 ePAC can mix remote and distributed I/O on the same network. 

Schneider Electric Modicon M580 ePAC

By default PAC’s interoperability and flexibility are much more apparent than a PLC, data exchanges between multi-brand devices and multiple domain applications like motion and process control are achieved on a single platform, this is a key differential compared to a PLC. Using Ethernet PAC’s can send data and receive information from other PAC’s leading to a master driven distributed control system (DCS) of connected PAC’s.

Application

PLC’s are ideal when considering smaller scale automation tasks, like single (on) machine control and smaller scale building automation such as, lighting. It’s not true to say PLC’s are fit for only this purpose, but for stand-alone use PLC’s are a natural and cost effective choice.

They will continue to be used extensively within industrial automation moving forward. PLC's are a main part of the industrial architecture and a lot of resource in both time and money has been invested in them. With programming know-how still dominating the industrial user base and continuious product development, they are not disappearing any time soon.

PAC’s are targeted towards more complex and larger scale automation architectures. They are particularly suited for multi-domain monitoring and control. Functions such as; advanced process control, motion control, drive control, vision applications and HMI's can be ran on a single platform, whilst using PC-based software to program, monitor and collect data.

They also handle much more distributed I/O when compared to PLC’s and tend to have larger memory capacities, making them suited for full plant control. Communication and integration between machines is the real selling point, they handle digital, analogue and serial signals as well as running intricate PID algorithms commonly used within temperature control.

Fast Forward

Technology innovation such as cloud based solutions and the Internet of Things (IoT) will ultimately make PAC’s a more attractive proposition. For manufacturers it’s important to stay ahead of the curve, faster, leaner processes allow for higher productivity rates and lower costs.

Employing the right platform and hardware must be taken in to account, but the choice is not clear cut. What one perceives to be a good solution, might not necessarily be the best. There are clear arguments for the use of PAC’s and the use of PLC’s.

For example a highly regulated site that needs tight control and employs systems such as; SCADA (supervisory control and data acquisition) and MES (manufacturing execution systems), will favour PAC. However within these sites it’s not uncommon to find stand-alone applications, where using simple PLC based solutions will offer less cost in maintenance and overheads.

End users are the most important aspect when manufactures are designing product. They are the ones who demand greater functionality across their platforms, whether that be PLC or PAC orientated control.

Please add your thoughts to this, also what be your reason in choosing one over the other.

Key brands

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