Strengths and weaknesses
PLCs were developed specifically for the manufacturing plant: dedicated to a limited set of tasks, they’re rugged, resistant to the rigours of the factory floor, such as vibration, heat and dust. “PLCs are reliable. They do their job well,” TechCold’s Adsett says.

A PLC’s main advantage, though, is not so much that it’s fast — today’s PCs have processing speeds that are orders of magnitude faster — but that its simple operating system (OS) means its main processing power is always available for the unit’s main task, explains Nicolas Arel, a systems integrator based in Montreal. Windows, the most popular PC operating system, demands priority on the central processing unit’s attention. In other words, if the machine that the computer  controls sends a signal at the same time that the OS sends a routine instruction to the CPU, the OS instruction will be processed first, forcing the machine’s signal to wait. If you’ve ever been frustrated by the hourglass symbol when you’re doing fairly simple work on your PC, imagine if that happens when a critical alarm signal from a manufacturing operation has to wait for the OS to refresh.

Worse, conflicts for computer processing cycles from different processes can cause the computer to crash, which can halt production lines or critical operations and even put people at risk if the system is controlling a safety-related process.

Because PLCs have been around for more than 40 years, their use is well known in manufacturing settings; programming them is relatively simple, and in most manufacturing plants, electricians know how to set them up and troubleshoot them.

But their functions are limited, like their programming language. They can handle input and output, but complex tasks are difficult to program using ladder logic. Setting up different responses to a wide range of circumstances that might occur is far easier in one of the richer, more modern programming languages used in personal computers.

Another limitation is that PLCs are proprietary systems; while manufacturers can choose from a wide range of suppliers, Allen-Bradley’s PLCs are not compatible with Omron’s or Siemens’. This limits the manufacturer’s choice and flexibility.
The personal computer, however, is (if anything) flexible. A multi-purpose device, it’s well suited to handling a wide range of tasks and integrating data in a way that’s easy to understand.

And while PLCs are well accepted and widely understood on the factory floor, PCs are equally well understood in the office. PCs are good at linking information from the factory to the front office. They’re often used for monitoring efficiency and displaying activity to management.

Today’s PCs use very powerful computers with extremely high data speeds compared to PLCs. They can handle images at high speeds and can be made compatible with many other computers, networks and even PLCs. And because they’re so widely known, there are a lot of people available who know how to program them.

But PCs are not typically rugged systems; they’re susceptible to heat, dust and vibration and have to be “hardened” or “ruggedized” for use on the factory floor. They’re also susceptible to computer viruses (something PLCs don’t have to worry about) and being hacked. And worst of all, they’re prone to crashing. This can lead to loss of data, which can be a huge problem and cause expensive delays when the PC is controlling critical manufacturing operations.

“PLCs are good for interlocking operations, and fast decisions. They’re not so good with high-speed data collection or complex programs,” Adsett says. “PCs are better suited to monitoring, optimization and analysis — making sure you’re getting out of a process what you should be.”