Fixed versus Modular PLCs 2

[oddmanout]

I am currently taking a course in PLCs. Would someone please define the terms "fixed" and "modular" PLCs? What are the major differences between the two?

Thank you.

 

[dV8r]

It will not take you much longer to just read your assignment, and then you will have the answer expected on your test instead of mine.

 

[jraef]

Fixed are fixed. Modular are, well..., uh, modular.

 

[ozmosis]

Reminds me of a job I was involved in a few years ago. Somebody was swapping over an S7-300 PLC with a new one. Our installation guy removed all screws and tried to take it out, it wouldn't come. I asked "are you sure you've taken all the screws out?" "yes, course I ******* have!" A lot of scratching of heads, furrowed brows and then realised the PLC had been glued on with something that was similar to an epoxy resin. This was definitely fixed, and probably still is somewhere in the skip we had to dump it.

 

[Skogsgurra]

That's a good one, Patrick!

Also, if a PLC is broken and the pieces are spread all over, I think that could be called "modular" - in a way. And if it is then (against a few odds) repaired. I suppose you can say it is "fixed"?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

Gawd Gunnar...

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[oddmanout]

Thanks for your help ... NOT!

If I had found the answer in the course materials or on the web I would not have posted the question on this forum. As it is I haven't found it here either.

 

[dchartier]

Hello oddmanout;

Sorry about your frustration, but...
If you can't find ANY answer on the web, and I guess the web is the greatest trove of dissiminated knowledge ever built... Maybe the original question is inadequate. Maybe that's the way it was written out in your manuals, but I can assure you that the dichotomy fixed/modular does not exist in the world of real, professional PLC programmers. Maybe you could go back to your teacher and ask for more explanations.
The names and distinctions between PLC types is more a matter of marketing by the manufacturers than a real naming convention.
Sometimes we will speak of "bricks", small PLC units that combine the processor and some inbuilt I/Os. Generally these are small-application oriented, small-memory PLCs: Siemens S7-200, Omron CFQ1, and others. But nowadays you can add I/O modules (digital and analog) and communication modules to these bricks to extend their range, so does that make them modular? I won't event attempt to answer that one.
Even the lowest usable Programmable relays (Siemens LOGO!, Moeller Easy, Rockwll Pico and others) come now with expansion modules of some kind, and their processing power is increasing, making the distinction between small PLC and Programmable relay less distinct than it was 5 years ago.
Sorry for the rambling, but just keep in mind that the peopo=le here will probably give you the best answer they can to a question that they can answer.
And your question has none, in our world.
Regards,
Daniel Chartier

 

[Skogsgurra]

We sometimes get so frustrated from not knowing the answer to a question that we just go nuts... Sorry. No harm meant.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ozmosis]

I didn't mean to be frivolous, I thought jreaf had answered the question.
I don't think you can get a 'fixed' PLC these days. I can't think of any that are not able to be expanded into a 'modular' format. In the old days you did have 'bricks' that couldn't be expanded and therefore the PLC processor only saw it's own I/O and nothing else. Modular in my book means having a PLC with a processor that will recognise and control any additional hardware connected that performs I/O functions similar to the base model.

 

[Skogsgurra]

It is also possible that you are asking about "Modular Programming" vs "one program does it all" programming.

The latter method is a natural (and primitive) way of writing PLC programs. You more or less write down everything that shall happen in one solid chunk of code. The former method is a little like Object Oriented programming where most tasks have their own (reusable) modules written for them and these modules are called from a main program with the necessary parameters.

Is that what you are asking about?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[skills]

A Directlogic DL105 might be the only seriously fixed I/O system that is around anymore, it provides no options. The DLO205 is an example of modular type design.

 

[itsmoked]

Would someone please define the terms "fixed" and "modular" PLCs? What are the major differences between the two?

Fixed: What you buy is what you get. No additional modules can be added over the native functionality.

Modular: You buy a basic unit, often just a "processor module" you then add what functionality you need by adding more functional modules.

To be called a PLC in industry now, the PLC must be, by definition, "Modular"!

So the major differences are:
Fixed: Don't exist.
Modular: Exist in profusion.

Any other question?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[PNachtwey]

This meaningless question is asked every year by HNC instructors of their HNC students. The answers are meaningless also. It is wasting time learning trivia when you should be learning more import things. Like Boolean logic, K maps, DeMorgan's theorems, state machines, Mealy and Moore diagrams etc.

PLCs are just tools and they will change. What we call them, what they can do and how they are programmed will change. When they become obsolete your knowledge about them will be obsolete. I would concentrate on those topics that will be useful forever.

 

[ScottyUK]

Star for that last post. The underlying principles are by far the most valuable subjects to be covered at college or perhaps later at university.


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Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[Skogsgurra]

I agree fully. But, trivia is also important. Think about it. Setting up and using a Karnaugh map can be learned in an hour. But that does not make you fluent in either logic or PLC applications.

It is all these less important "trivia" (I could mention a couple of dozens) that finally makes you productive as an automation engineer. The fact that you do not have to ask "meaningless questions" all the time is valuable as such. Even if the real value of every single piece of knowledge is small or non-existent.

BTW, I know very few people in the automation business that know about Karnaugh, Quine-McCluskey or Mealy. They just do their work - mostly using their very low-level knowledge about ANDs and ORs. Even XOR is a mystery to most of them. Some use Finite State Machines, but do not know that they are using them.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[controlsdude]

Here is my two cents

Modular PLC - PLC that has is racked based and has slots for individual cards and the processor.

Fixed PLC - any PLC that is a brick style, and it does not matter if you can add more options. Its still a fixed style to me in the basic configuration.

 

[dV8r]

The key word here is "logic". Some people can "think logically" others stuggle with it.
To work with PLC's you only need to know "10" things, they are "ON" and "OFF". Now think logically and control them, VERY simple.

 

[ScottyUK]

God help you when you find a PLC made later than 1980... all the confusion of the analogue world awaits...


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Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[dV8r]

All control of the analog in a PLC goes through a AD coming in and DA going out. Or have they finally found memory to hold analog signals?