November 23, 2014
 
 
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Automating Manufacturing Processes with Ethernet-Enabled I/O Modules, Linux

What's A PLC?

  • October 9, 2002
  • By Rob Reilly

Traditional control of industrial processes has been carried out by small special purpose-built microprocessor based systems called PLCs, also known as programmable logic controllers. They have very fast cycle times and essentially run in an endless loop looking at inputs, making decisions and controlling outputs. PLCs are programmed with an industry standard language known as ladder logic. They can respond, in real time, to hundreds of inputs and can control all types of motors, relays and actuators. PLCs are the quick thinking brains of the industrial automation world.

As the old hot rodding saying goes, "Speed costs money, how fast do you want to spend?" PLC systems are plenty fast and plenty pricey. In the case of PLCs all that speed also needs to be protected from the industrial environment. The control computer (PLC) and I/O modules have always been located physically near the industrial machine in a metal cabinet, right out there on the shop floor. Putting the cabinet next to the machine being controlled made sense because all the switches, sensors, motors and relays had to be connected by heavy wires back to the I/O modules.

But what happens when you're going to design a manufacturing process that doesn't need insane real-time performance? Also, what if you want to collect a whole lot of data, do some calculations on that data, and then automatically send the results (charts and graphs) to various people in your organization, automatically? Finally, what if you wanted to have the control computer (desktop PC) located in your programmer's office and out of that industrial shop floor environment?

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