Hints To Program A Plc

[VILMENAY]

I want to know if exist a rule of thumb to design, develop and organize a PLC program based in either the functional specifications of a new project that will be controlled by a PLC or just an upgrading of an actual system controlled by "relay logic" that we'll obtain the functional spec's from it's relay ladder diagram. Exist a difference in the approach to be used for the two cases mentioned above (new project and existing project)?

 

[Allen Nelson]

• Plan
• Organize
• Look at some stanards (GAMP, S88). They don't tell you how to code - they tell you how to lay out a project
• Modularize
• Think before you do.

There's no silver bullet, or "Universal PLC code" (although I'm working on it).

Unfortunately, experience helps. You learn the mistakes to avoid. This frees you to make all new mistakes.

 

[Steve Meisel]

Allen,
Where can I get a copy of GAMP and S88?

 

[Terry Woods]

Certainly follow what Allen said. In addition to Modularity (one of my favorite bones to chew), develop your program from Top/Down AND from Bottom/Up - Concurrently!

One of the harder parts is determining your modules... where does one end and another begin.

It's easy enough to say... "This section of the machine is one module, this is another...." and so on.

I find that modularity is more a matter of function than location. You need to recognize that while there are mechanical functions, there are also, and more importantly, "conceptual functions".

A conceptual function would be something like... "Go To The Store".
The mechanical functions associated with that concept involve things like... stand up, turn left, walk down the hall, open the door, etc.

The mechanical functions are a sub-set of the conceptual function.

Now, if you think about it, those mechanical functions are really other conceptual functions! What does it really take to stand up? It takes a lot of mechanical activity and coordination!

As you continue developing you'll find that you need to receive, and sometimes, transmit messages.

Before you execute "Go To the Store" you'll probably need more info from the Wife-Module. Besides telling you to get up off your a$$ and go get some beer, the Wife-Module might also tell you that there is a list in your pocket.

You need to plan what messages are needed by which modules, where those messages are going to come from, and what messages are sent by modules and to whom.

I don't know of any standard, or any book for that matter, that has been written describing a rational approach to these things.

In general, it boils down to you paying close attention to the details and organizing YOUR understanding of those details.

A PLC Program is nothing more, or less, than Organized Details!

 

[ganutenator]

I loved that, Terry

I love the Wife-module concept explanation. May I use this?

 

[Allen Nelson]

GAMP - http://www.gamp.org
S88 - http://www.isa.org

I beleive both organizations expect you to pay for the standards. It's certainly true for ISA, but there's a minimal charge for ISA members.

You may want to do a bit of GOOGLEing to determine if you want these. As I said, they are less about PLC coding and more about organizing the whole of the project.

GAMP is big on the documentation trail. S88 is one method modularity that Terry & I mentioned.

 

[Terry Woods]

Most of us already have a Wife-Module, so.... I guess it's free territory... although, if you make a buck usin' it, could I have a dime?

My kid calls me and my wife the Parental-Units.

 

[Terry Woods]

Have any of you noticed that posts that you reply to disappear?

Not the reply you made, but the post you replied to!

Uhhhh, Phil? I think there's somethin' amiss here!

 

[Terry Woods]

Now they're all back! Hmmmmmm....

 

[Allen Nelson]

Actually Terry, I've notice whole threads disappearing, permanently.

Remember something from HIGHVOLTAGE, the IT interviewee who wanted to know about HMI tags?

Not that I'm sorry he/it's gone, but....

 

[Phil Melore]

Hi,

Terry:
After you make a reply, trying going back to the table of contents page. You should see the post you replied to there. If not email or PM me... but I think that's what you're seeing.
Maybe I should make the page go back to the TOC instead of the post just made... hmmmm there's something for my to do list. Thanks for the input!

Alan:
Yes, the HIGHVOLTAGE hmi thread has indeed disappeared. I got quite a few "concerned email notifications" and "I'm never coming back until you..." and "why don't you moderate better" emails about that thread. A new record, actually.
Unfortunately, I didn't check email until around 10 or 11pm my time so it was alive almost 24 hours. After reading it, I honestly couldn't see a reason to keep it as no knowledge was shared and tempers were flaring. It did make me realize some new words to censor though...

So please, all you fine folks out there, "can't we all just get along"?

Enjoy,
Phil

 

[rsdoran]

Phil, if I may say so the temper was flared at me I think. There was some things being shown there..I think so anyway. I myself have used this forum not just to learn about plc's but to learn how to "ask a question", and "answer a question". Some questions need blunt kind of answers. I dont mind the anger etc because it also teaches me to learn temperament. I was blunt in my answer which angered Highvoltage and caused a sarcastic reply. I can deal with that, to be able to assist you must be able to get the person's attention.

Anger, love, hate are all emotions. Emotions create a drive that make people learn things they are passionate about.

If its possible please put the Highvoltage post back on, if we (you) remove any post that has arguments then there wont be much to learn from. The tangents/arguments really bring out the best in most.