New video: What is a PLC?

Programmable logic controllers — or PLCs — are microprocessor-based electronic components that serve as the programmable smarts for simple or isolated applications. Now they’re also finding increased use on integrated machines and more involved automation setups …

PLCs came to being in the 1960s to replace relay-based controls — whole cabinets of relays and impressive runs of wiring in some cases. This trend followed the electrification of manufacturing — and the increasing complexity of design requirements. So traditional hard-wired relays have largely yielded to reconfigurable PLCs. Now, it’s true — primitive hardware and rigid task execution were initial PLC limitations. After all, early PLCs were limited to the electronics of the day.

But as PLCs evolved, they came to have unbeatable reliability. Today, their logic and memory boards, backplane interfaces to I/O modules, and generally heavy-duty circuitry help make PLCs indispensable.

The basic parts of any PLC system include the processor, I/O modules to handle inputs to the controller and outputs to the controlled devices, and some type of user interface which could be as simple as a keypad or a touchscreen interface or a programming link via a PC. The PLC’s processor is programmed via the user interface. The I/O modules are used to bring input signals into the PLC’s CPU and output control signals to controlled devices such as motors, valves, sensors and actuators, among others.

One core PLC strength is how they let engineers write application code to suit specific applications in which they’ll operate — on assembly lines and in machinery and elsewhere on factory floors … with other electrical, mechanical, and electronic equipment. Here, the original form of PLC programming known as ladder logic  — finding its roots in good ol’ relay logic — is still dominant.

Rumors of the PLC’s demise have been greatly exaggerated. Despite industry predictions to the contrary, PLCs are still by far the most common choice for standalone or single-axis motion. Such applications are seeing more inroads from motor drives sporting controller functions. These motor drives excel in machine designs that still need PLC functions with multiple interfaces — including Ethernet communications and digital I/O, for example. Because such motor drives can also incorporate motion controls delivering S-curve, camming, and freeform motion profiles, OEMs are more likely to pick them than PACs in otherwise-simple designs eliminating standalone PLCs.

Part of the PLC’s enduring use is how their cost — at least on the hardware side — has consistently gone down over the decades, even as their capabilities have grown. Some PLCs are just a few hundred dollars. One caveat here is that PLC software sold through license agreements can increase overall design costs.

In addition to ladder logic, some PLCs can be programmed in higher-level languages such as C and BASIC or run proprietary languages. Many offerings aim to homogenize industrial PLC programming, and all aim to reduce work for end users. For more videos like, this visit

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