Interesting question.
Why do we use PLC's now?
I agree that anyone can put a uC, a crystal, a few optocouplers and relays in a PCB, and you might think, eureka, why am I the first to discover this?
I've developed projects professionally using PLC's, I've used OMRON, SIEMENS, MITSUBISHI, YOKOGAWA, TELEMECANIQUE, TOSHIBA, AB and FANUC and I'd like to share my opinion.
First, what is the point here? is it price? ok, uC's are cheaper than PLC's, I agree, but, let's say that I want to use my microcontrollers, instead of buying someone else's PLC's, I won't be able to design and make a specific unit from one day to another so it should be convenient to stock a few of my uC's ready for any application that should come up.
- I must take some time to design, debug and test, and test again, the hardware, software and firmware for the basic CPU.
- Same with the product itself, which includes the plastic casing, for a decent multimodule PLC a mounting rack, even the packaging box.
- Now let's move to the modules, to have a competitive brand I must carry many useful modules, DC input, AC input, relay output, triac output, transistor output, analog voltage input/output, analog current input/output, temp (TC, PT100) input, RS232 comm, RS485, CAN, PROFIBUS, more recently ETHERNET, wifi.
- I must design a handheld programmer for my product (to stand beside PLC's brands).
- I must provide my product with a complete family of HMI's.
- Finally let's don't forget that each and everyone of my products must comply with various international standards, like UL, CE and FCC.
Until then, I will be one of all the fools that buy PLC's to someone else, instead of making their own, yet the amount of time and money it'd take me as a person is quite high.
So, I think, using someone else's PLC saves time and money*.
Even so, you will never get rid of using someone else's platform, many times you will have to troubleshoot an existing system and so, you need to have the softwares and the cables for various brands; if there is a damaged PLC in a network, you have to replace it with the same model PLC, not your uC; some big customers** will ask you for a specific brand of PLC, either because they like it, trust it, or they want to keep uniform their plant's PLC's.
Personally I use PLC's and uC's, everything depends on the application isn't it? now that i've thinked about it, the question seems a little bit superficial.
stan4.
* If you manage to grab a distribution from most brands, they will give you a percentage of discount from 20 to 50% of the list price.
** Coca-cola requested for a SIEMENS PLC in a recent project, I also noticed they had over 90% SIEMENS PLC's. Nestle requested for an AB PLC in a recent project, the maintenance manager told me they had an alliance with AB.