STUDENT QUESTIONS, please post them here!

[Terry Woods]

Thanks, Dan.

 

[jstolaruk]

When I looked at Terry's code for first time I was a little puzzled as to why the high level switches were tied into the low level. After he explained it I can see the logic and wisdom in the design.

Yeah it is a little more complicated and it might just confuse the electrician who is more interested in his newspaper. Speaking as an electrician
1. What is he doing reading a news paper on the complany clock?
2. If he does not want to advance his ability then maybe he better stay with the newspaper or better yet the TV.
3. He seems lazy and determined to stay ignorant and incompetent.
4. I would keep him on - but for changing lite bulbs maybe.

Dan Bentler

This is the way it works in most auto plants employed with UAW help. Skilled tradesmen have job descriptions that dictates their duties and they are not to be doing anything else. So, if they are reading their paper its interpreted as a good thing because that means everything is running.

 

[leitmotif]

I have also worked as a standby breakdown electrician and sat in the shop. Yes I have read the paper on the company clock too so I must limit my pomposity. Trouble is I get quickly bored sitting around the shop and the paper is only good for about 20 minutes so I look for other stuff to keep myself occupied. Guess that is why I am still going to school and learning new things (at least things new to me ??)

I WANT
The cops able to sit around and drink coffee and eat donuts
Fire men to sit in the station
electricians to sit in the shop.

If they can justify it then they have done their job correctly.
BUT I also want them learning new and better ways to do business not doing mental stagnation.
Dan Bentler

 

[jstolaruk]

leifmotif, I agree but the reality is that you are one of the exceptions. Training is encouraged but often its on their own time.

 

[Lancie1]

Terry, this is a good example of looking at the "causes" but not considering the RESULTS, the "outputs".

However, is it simply the action of the Lower Float Switch that causes the pump to come on?

That better not be so!

The TRUE "cause" for starting the pump should be "Tank is Low". (This is not to be confused with the input signal "Level is Low")
Once the pump is running, the TRUE "cause" for stopping the pump should be "Tank is Full". (This is not to be confused with the input signal "Level is High")

On the other hand, if you had started with the OUTPUTS, you might had said, "Okay, I got a PUMP (an "output"). What are all the things that need to make it STOP?" (Insert your great treatise on Level Switches here). Also insert some STOP pushbuttons both local and remote, maybe an Emergency Stop button, etc. Now what are all the things that make it START. Huh? I suppose I need a Manual Start Pushbutton, maybe an Auto/Test Selector switch, and those same Level Switches, maybe an alternator circuit if there are two pumps that work together, etc. Some of these things do not even get mentioned when thinking only about CAUSES. To write a program efficiently, you have to consider the final DESIRED RESULT, as well as what you mentioneed, the causes of various conditions.

 

[testsubject]

In all this, I hear a lot about the maintenance technician being unable to understand a complex program. To be honest, I do not want anyone in my program except for me. It is a complex piece of machinery and I know it the best. I do document as well as I can, (still improving on this). We do not provide a hardcopy or a disk with the program on it with any of our machines. What with a touch panel, serial communication device, and the Plc, there is a lot that can go wrong if someone inexperienced decides to see how it works.


Terry,

You can count on me being able to understand what you are trying to say. After all this time, I value your posts. I find them to be very informative.

Bob

 

[jstolaruk]

It certain depends on the circumstance of what your repsonsibilities are.

For the in-house programmer whose function is to maintain that machine, that kind of makes sense. Though from experience I can tell you that it is often frowned on by management. They don't want to have to rely on one individual. What happens if you leave the company or worse, you are unable to work because of disability or death?

For the machine builder, it depends on what the contract is with his customer. If the customer is paying for the design as a deliverable, electrical and mechanical, then its the machine builder's responsibility to provide a machine that can be maintained by the customer's personnel. It's in both parties best interest to make it seamless. I know I don't want to hear from the customer after the installation that something is wrong and I have to go for a unbillable service call. And the customer is a whole lot happier if problems are remedied quickly by his in-house support staff.

If the contract is for a "standard off-the-shelf" machine and the customer is not paying for the design, then the code is not a deliverable and service calls would then be billable. The downside: customers have memories like elephants.

 

[davidg68124]

In all of this, I have to agree with both Terry and Bob. I have been taking courses on advanced logic design, data structures, high level mathematics, and programming. In these courses I have learned to see my mistakes in the past. It just has served to make my programs more 'bullet-proof'. No gotcha's or hacked solutions. I have learned to make my logic more predictable and therefore, more friendly. Just because we go to the lengths to design a system that is more logically sound, dooesn't make it wrong. I have noticed as of late, the inability of people to think logically. Currently, I am working on a project that OEM vendors are producing ALOT of that HACKED software. They obviously had a problem and beat it to death with poor programming practices to finally get it to work. Mind you, this this is a $4.7 Million Dollar piece of equipment. Their HMI uses nearly 10K points!!! Whikle this equipment is important, I do not think it warrants using more points in an HMI than the last 3 PLANTS that I have worked in.

I have always prided myself in providing elegant, yet simple solutions. Only to find after last semester, I still was making errors. But I am always advancing.

Today's equipment is more complex and therefore requires better programming efforts. You do not want to be responsible for a bug that could have been easily avoided with proper design technique, that ends up costing a company a Million Dollar batch! Today's maintenance tech is really not capable of maintaining the type of equipment that is being produced. They do not want to take the time it requires to keep up with technology. In most cases that is. There are some that can and do. But they end up moving on into better positions.... Sometimes like us...

David

 

[bwtaylor]

I'm about to purchase a PLC Training Program. I already have the PLCS.net material, thank you... but, since I'm being laid off, my company wants to spend $5K training me - plus I will get a $3K Displaced Worker grant.

The point is, I can use the money for training, or lose it.

I already have an electronics associate degree, and am aware of the need to coninue toward a BS. That's not the issue. I want to get some PLC training.

I'm looking at:
George Brown:The City College - PLC Technician Distance Learning Program
http://www.plctechnician.com/
...and...
Industrial Text & Video
http://industrialtext.com/index.htm

Which would you recommend? Either? Neither? Any suggestions?

 

[Lancie1]

bwtaylor,

The George Brown course will give you more theory and more details, but you will not get a feel of the hands-on experience that the Industrial Text videos might have. The George Brown course does include a PLC Simulator program, but you can get that for $40 from The Learning Pit.com, as opposed to $435. If you really want to learn PLCs, a better choice for PLC training would be your local technical school or community college. If you just want to spend the money because it is there, then the on-line courses will be easier.

 

[Bob O]

I have never used it but I know it comes highly recommended.



http://www.thelearningpit.com/

Bob O.

 

[rsdoran]

$5000 is a good piece of change to have available. Since you have an electronics degree you may want to look into certification and going to hands on classes. I dont think much credit is given to online classes yet.

Allen Bradly offers "vouchers" where the 5k could be put up and you can choose when to take classes and testing. It also offers options in other areas of automation.
http://domino.automation.rockwell.com/applications/gs/region/gtswebst.nsf/pages/Training

AB isnt the only one too offer this either.
Microsoft, networking or programming certification are also good options.

Check with the local suppliers to see if they offer training classes.

 

[bwtaylor]

You guys are the best!... and PLCS.net is the place to be.

I'm familiar with the certification process (A+ and Network+) with more
on the way. I did not realize that there was a certification for PLC.
This will lend a considerable amount of credibility to my resume... plus, I really want to learn.

Thank you, Lancie1, BobO, and rsdoran.

Anybody with 2 cents to add, please speak up. This is, after all, the Student's thread.

 

[Terry Woods]

David,

I like what you said.

I "learned" PLCs long before I went to college for my EE. I "learned" from the AB Books... thought I knew all there was to know about those new fangled PLC-thingees. They were simple as far as they went... and as far as I could see. It wasn't until I got to the Digital Design Course in college that I found out just how far off base those books really were.

It's not that the PLC books told me anything wrong... it's just that... well, they are supposed to be about Programmable LOGIC Controllers... they do cover the programming part, that is, how to use and enter a particular instruction... but they don't explain anything about LOGIC other than AND and OR. And they sure don't explain anything from a "System" point of view.

I guess they felt that they needed to keep it simple for the simple-folk.

It's kinda like deciding that you want to be a carpenter... so you run yer butt down to the Technical Book Store and buy a book on "HAMMERS". The cover, Page-1, shows a shiny new hammer. The top of Page-2 shows a hand holding the "holdin' end". The bottom of Page-2 shows the "bangin' end" hitting a nail. Page-3 shows the "pullin' end" pulling out a nail.

The sentence at the bottom of Page-3 says... "CONGRADULATIONS! You are now a bono-fide hammer-holdin', nail-bangin', nail-pullin'... hammer-swingin' kinda guy! Go get 'em TIGER!" (Page-4 is blank... how much can you say about a hammer?)

Right... as if now you're qualified to go build another Sears Tower.

Most PLC books really aren't that simplistic, although some are... but most of 'em ain't much better than that.

The bottom line is... the books from PLC Manufacturers do not teach digital logic, nor do they teach digital design, and they sure the hell don't teach system design. They only explain the particular functions that they include with the particular PLC. As long as a particular application stays exactly within the boundaries of the included examples then fine. However...

The best example I can think of to illustrate where a fundamental logical operation (a fundamental logical concept) is applied in a manner that is so limited as to imply that there is no other use for the concept, nor any other variations of that concept, is the AB MWM (Move with Mask) Instruction.

The MWM Instruction provides for particular bits of a source word (selected by the 1-bits in the mask) overwriting the bits of a destination word.

That's it. That is the full extent of "masking" as far as they are concerned.

What a lot of AB-guys don't realize is that this is a particular function specifically created by the AB Engineers to perform the indicated task. Conceptually, this is a MOVE WORD operation from one word to another. However, the word moves "through" a filer, a particular type of mask. In this case, the 1-bits are "passes" while the 0-bits are "no-pass".

The real "logical operations" occurring here is an "AND" operation followed by an "OR" operation. The Source is "AND"ed with the mask and the result is "OR"ed with the destination.

There are are many other situations where "AND", "OR", "XOR", "NOR", or "NAND" might be used in any order in the two-step operation to produce a move-with-mask result. When and why would any of these variations occur? It depends... situations are different.. all the time. It ends up being the same sort of thing... a move is made through some kind of filtering operation.

Even though these variations might not be available in a particular "custom made" function, they are available through the implementation of the fundamental logical operations available in all upper-level PLCs.

The point is, PLCs are much more capable than the manufacturer manuals indicate. It is only with an understanding of the Fundamentals of Digital Logic and an understanding of Digital Design that those capabilities (assuming that the particular PLC has the full set of fundamental capabilities) can be realized.

But... and this is a big but (no, not a big-butt ...just for Eric)... none of that matters if the programmer doesn't have a "Sense of System". Without a "Sense of System", a programmer is nothing more than a hacker.

 

[testsubject]

Thanks David.

I am quickly finding out how much I don't know about digital logic (and I have been doing this for almost 10 years now) and how much it has hurt me in wasted time by not knowing.

Oh, yeah. and ditto on everything Terry said...

Bob