What is a PLC?
I'm going to start by giving away the ending: PLCs are all about reliability.
A. They must be fault tolerant in their target (industrial) environment, both electrically and physically
B. They must be supported by robust logistics - supply, technical support and ecosystem, delivery, service, and parts aftermarket.
They are NOT about capability, programming language, nor speed. Generalized computing technology long ago made all those considerations pointless to argue. Your iPhone will outperform IBM mainframes of quarter century ago, and outperform all the compute combined of a half century ago. You knew that. And that leaves only price performance... which is being swept away every minute. You knew that too.
So if you wanted to build PLC based on an Arduino or Raspberry Pi, there is nothing technically stopping you. But it had better be reliable (better not blow up, short out, fry, or succumb to a strike or drip). And you had better have logistics behind it.
My corollary point herein is that reliability has gotten cheap too. And then the only barrier left is logistics, and that can only be addressed by time and business models. Maybe a few years from now Koyo will have changed that equation from below. Or Rockwell will have solved it from above.
So if one were to build a PLC, and were willing to support logistics by only one's own guile (dangerous), one could choose an ARM or x86 core on a cheap board at under $10 for the brain. The choice should be one with the broadest support community, and should have isolated and fault tolerant I/O. Good examples are Maxim's PLC reference designs (like MAXREFDES#64). Note their use of the ST STM32L and STM32F series.
- ARM or x86 cores on a cheap board at under $10. Pick the most broadly community supported.
And consider well
- capacitive couplers and fault tolerant communications transcievers
- Ethernet both (isolated and ubiquitous)
- isolated 5VDC USB power (also ubiquitous).
AND package in DIN mounted cases, metal if you can afford it. Shield it per FCC, CE and UL. Use screw terminal connectors (or other industry standards). And consider IP69K type access protection: "Good for ten years under salt water to 300m..." - Okay, a little hyperbole, but take offerings from the likes of Rockwell and Koyo as your models.
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