microcontroller vs plc vs 'microprocessor w/ peripherals'

i worked as a technician for a japanese company that shall remain undisclosed but anyway in the machines in their assembly line i saw on a machine that it had more than one plc(programmable logic controller) in it, as well as a computer but with no monitor (machine just interfaced to the computer via a port) and i dont know if it had any microcontroller chips in it.. but anyway i have a few questions

why would a control system need all of these? why cant one control the whole thing? why more than one plc? what can a plc do that a micro or a microcontroller or microprocessor can't? and vice-versa? all i know is that a plc is usually more expensive than a microcontroller so what exactly are you getting for that extra you're spending?

Without more details on the application, multiple microcontrollers can be used in a modular approach. One microcontroller per task; it makes for simplified programming = lower costs.

There are many other advantages: you can replace one defective module, no need for expensive high-end multi-feature microcontroller, easier to add future modules, etc.

I always design in a modular fashion. It's so much easier to program, debug and repair.

Robert

(I have no experience with PLC)

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Originally Posted by Demon

Without more details on the application, multiple microcontrollers can be used in a modular approach. One microcontroller per task; it makes for simplified programming = lower costs.

There are many other advantages: you can replace one defective module, no need for expensive high-end multi-feature microcontroller, easier to add future modules, etc.

I always design in a modular fashion. It's so much easier to program, debug and repair.

Robert

(I have no experience with PLC)

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ah yes modular design would be a perfect reason to have multiple 'brains' handling control rather than one that is centralized... makes sense for industries

Many machine builders use PLC's for there logic control because they are mass produced and usually available on quanity's. Also PLC's are already designed which allows the machine builder concentrate on the machine mechanics instead of designing new circuit boards and logic and interfacing. Most PLC's use LADDER LOGIC for programming which allows PLC software engineers to understand each others software.

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Originally Posted by Humbleworker

ah yes modular design would be a perfect reason to have multiple 'brains' handling control rather than one that is centralized... makes sense for industries

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makes sense for Hobbyist as well, if you can use 2 $0.49 PICs instead of an $8.00 PIC too. One other advantage is you don't have to concern yourself with how timing of one function affects the other function.