PLC or microcontroller?

hello,

pls, what could be the difference between a PLC and a microcontroller. why would somebody want to use microcontrollers(i.e PIC) instead of PLC or vice-versa.

Hi
Microcontroller is basic structure of PLC
Take example of 8051 its having 4 ports you can assign these ports as i/o
Uc use for small application
PLC are for industial use

Sandymax

Hi,
Microcontroller is a simple IC, and the PLC is a complete industrial device.(However, both of them are programmable.)
If you want to use a microcontroller, you have to build a printed circuit board for it.
The difference between microcontrollers and PLCs is similar than CPU and PC. PC is a computer and the CPU is only a processor.

"why would somebody want to use microcontrollers(i.e PIC) instead of PLC?"
Opportunities with a microcontroller are unlimited. And microcontrollers are much faster than PLCs. For example, you can measure microsecundums with a PIC.
Moreover, the PLCs are more expensive.

PLC's are micro-controllers which run closed loop really fast, and have a whack more Interupt routines.
I have pretty good experience in both, and find that it depends on the application. Usually cost, mobility required, and IO count determine which way I go

Microcontroller is a term coined for a microprocessor incorporating an I/O structure and an on-board memory. As such it has nothing to do with PLC's.

It is correct to say that Intel 8051 is a microcontroller.

It is used within the Micrologix PLC.

It is incorrect to say that a PLC is a microcontroller.

It is correct to say that a microcontroller resides within a PLC.

This is not so much an answer to the original question, but a question related to the same subject.

With the PIC, things are sequential, I mean if I am counting down during a timing sequence, I am not polling the inputs nor am i timing any other events. However, with PLC, there's hundreds of timer that can be timing in parallel and checking inputs for any changes at the same time.

does anyone have a "scheme" to multi-task in a microcontroller?

The big difference is that a microcontroller and its software/firmware tools are intended for general purpose electronic applications. Typically you buy a development kit, code and debug your source code on that. The actual product that you are going to sell has the microcontroller chip embedded in it, and at some stage in the production process the compiled code from the development kit is burned into the target micros.

The end product could be anything from any piece of consumer electronics, to some sophisticated gizmo buried in a piece of military hardware, but usually characterised by:

1. A relatively low I/O count.

2. No end user programmability, scalability or expansion.

3. Relaxed enviromental specs.

4. Short product lifetimes and spares availability, and zip to none aftersales support.

At the lowest sizes a small PLC and a Microcontroller are somewhat similar inside, but at every other point they differ. At the larger sizes, PLC's such as Siemens S7, Schneider Quantums and Rockwell's Controllogix systems are far more complex than any microcontroller.

Most especially the degree of software and firmware development in a full-scale PLC hugely outstrips what can be achieved with any small microcontroller, indeed the effort involved would not be so very much less than that involved in producing and maintaining something like the LinuxOS, or even WIndows XP for example. The biggest difference is that PLC firmware and hardware is designed and tested to be many orders of magnitude more reliable than any commercial OS such as Windows.

And from an end-user's point of view a PLC is characterised by:

1. Can be scaled and extended to many thousands of I/O and complex industrial devices.

2. The end-user had total programmability and application control.

3. Much tighter enviromental specs and packaging ruggedness.

3. Product life-cycles of 20 years plus are the standard in Industrial Automation, with spares and support generally assured in that period.

Is that enough for your homework?

does anyone have a "scheme" to multi-task in a microcontroller?

Click to expand...

You can purchase an RTOS (real time operating system) for
just about any microprocessor. It will give you a real time pre-emptive multitasking environment.
sharing data between tasks is fairly complex. You will need to study at least some of the techniques involved such as semaphores etc.

You can create your own multitasking environment in an assembly code. Use one of your timer/counters in the PIC.
Vector into an interrupt when timer overflows (standard techniques to create a timed interrupt).

Update your PLC type timers when in an interrupt.

I/O is usually scanned at the beginning of the main program loop. As your program grows it will be scanned less frequently.
In the PIC you could scan the critical I/O during the interrupt as well.

I like PhilipW's answer.

I don't

elevmike said:

I like PhilipW's answer.

Click to expand...

A microcontroller is just the brains of the PLC. It is not a whole PLC. It is a subset.

So how can a PLC have higher environmental specifications than the weakest component that makes up the system?

So how can the PLC have a longer life cycle than the microntroller that the PLC uses?

Orioginally posted by Peter Nachtwey:

So how can the PLC have a longer life cycle than the microntroller that the PLC uses?

Click to expand...

That's one of the nice things about a developed product. The manufacturer abstracts this from the end user. If the manufacturer wants to continue to manufacture the end product he needs to switch to a different, avaliable component. I would guess the company you are with needs to make this choice on a relatively regular basis, Peter. Do you continue to supply a product and redesign it to use the latest components or do you let the product die. I know the company I used to work for had to do this relatively often.

As Peter correctly pointed out a microcontroller is (possibly) just a component in the plc.

Keith

Peter,

In gernric terms, I'm presuming that the thread starter was refering to something like a PIC or Basic Stamp, since the question was, "what's the differance". It's a matter of the use of terms.

I think that Phillip's comments are correct in the way that the original poster is comparing a PLC with a PIC "kit".

I dont think the original poster meant "microcontroller" as in a specially developed CPU like the one that is the heart of most PLC's.

edit: Seems like I and Mike thinks the same.