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Fixed versus Modular PLCs
2

Fixed versus Modular PLCs

Fixed versus Modular PLCs

(OP)
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.

RE: Fixed versus Modular PLCs

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.

RE: Fixed versus Modular PLCs

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

RE: Fixed versus Modular PLCs

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.

RE: Fixed versus Modular PLCs

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

RE: Fixed versus Modular PLCs

(OP)
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.

RE: Fixed versus Modular PLCs

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

RE: Fixed versus Modular PLCs

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

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Fixed versus Modular PLCs

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.

RE: Fixed versus Modular PLCs

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

RE: Fixed versus Modular PLCs


   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.

RE: Fixed versus Modular PLCs

Quote:

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

RE: Fixed versus Modular PLCs

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.



RE: Fixed versus Modular PLCs

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!

RE: Fixed versus Modular PLCs

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

RE: Fixed versus Modular PLCs

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.

RE: Fixed versus Modular PLCs

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.

RE: Fixed versus Modular PLCs

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

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

RE: Fixed versus Modular PLCs

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?

RE: Fixed versus Modular PLCs

Jeez, does it? I had never realised! Thanks for clearing that up.

I can't think of any PLC where you directly interact with the A/D and D/A hardware, in fact the only way in is usually through the system register which allows you restricted access to certain parameters. All the ones I've worked on use a text or graphical interface to represent ladders and loops and screen the user from the hardware. Obviously you know better! smile
 

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

RE: Fixed versus Modular PLCs

Probably. There are such animals. Just google "analog memory".

But I doubt if that is what was meant.

(Now, I expect quite a few posts telling me how wrong I am and that I don't understand the subject)  smile

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Fixed versus Modular PLCs

I'm surprized that there are people using PLC's and do not understand how they work. All digital electronics are made up of two state devices either "ON" or "OFF". It is very simple, the complexities come in when one starts to string these together and cascade them by the millions. Or is it done differently in the UK?
And no, the analog memory device has not been perfected yet.

RE: Fixed versus Modular PLCs

Interesting that you think we have forgotten how the digital computer works. After all it was invented in the UK. Let me see... one, zero, yes I think I can follow that.

Any application programmer who uses the PLC does not normally write the application program as a series of 1's and 0's direct to the processor memory, nor in assembler code, nor even a mid- or high-level programming language such as C or Pascal. He uses the software from the PLC manufacturer or a third party vendor which presents him a graphical or text based interface which does the job of interacting with the digital world of the microprocessor. Have you ever programmed a PLC? It is difficult to tell from your posts.
 

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

RE: Fixed versus Modular PLCs

Is a capacitor not an analog memory?

RE: Fixed versus Modular PLCs



Oddmanout:
I like the way you defended yourself.  Did you notice that the sarcasm went away and people started answering your question?  Good reading for a not so informative question.  Either you textbook is out of date or you instructor has been out of the “real world” to long if he or she ever was. A confidant good defense is a trait that is not seen much anymore.  Those who have it go far. (Oh my God, I sound old)

RE: Fixed versus Modular PLCs


   Yes, Oddmanout, you showed great patience and restraint in your response. Good show.

RE: Fixed versus Modular PLCs

I'll bet I have a N.I.B. AB SLC-100 behind a ton of boxes, maybe I'll stick it in a museum.


RE: Fixed versus Modular PLCs

(OP)
I would like to sincerely thank all of you for your very informative responses. A lot of good reading here re: background information on PLCs that I was totally unaware of and is not mentioned at all in my course materials.

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