Simple Lader Logic Question! PLEASE HELP!!

[goatom]

Logic

Thanks again for all of the help. Yes the terminology for me has been the biggest wall, but as in anything if we stay with it, it will finally seep into the gray matter. I thought I was really understanding logic until I read that post and had to try and understand just what was being asked. What is still a bit foggy is the question of "show the state changes after 5 scans" In my mind after the first scan the state will be the same for the next 5 or 50 scans unless a command is given to change an input. This is what is giving me trouble. I will read up on the subject today and if I cannot get in pounded in I will post again. I know this is very basic stuff and I thank all of you for your patience and help.


Also, if any of you are having trouble with scaling go through Ron's work on the subject. It was awesome and was worth every minute of effort. Thanks again Ron for your efforts.

Have to run....my PLC trainer is smoking again. I think it was that 5th scan that started the fire.

 

[rsdoran]

I started to reply earlier, starter broke tho.

I thought there may be some confusion, actually think I wrote a reply but its gone, oh well.

As mentioned the rung starts with A off so B is off; therefore C gets turned on; which latches D; which turns on A THEN turns off D. E does nothing because E never gets turned on.

It starts with scan 1, so every even number scan A will be on, every odd number scan will be same as the first one.

 

[Ron Beaufort]

...unless a command is given to change an input. This is what is giving me trouble.

hint: in simplest terms, as you go down through the rungs, some of the OUTputs are being used to change the 1/0 status of some of the INputs ...

if you think that the only way to change the 1/0 status of a bit (either input or output) is by using some event from OUTSIDE the processor, then THAT is where you are being confused ...

specifically, an OTE or an OTL or an OTU (the things that you are apparently calling "outputs") CAN be used to change the status of any bit/box in the processor ... if those "changes" happen to affect the True/False evaluation of the XICs or XIOs (the things that you are apparently calling "inputs") then so be it ... life goes on ... more specifically, an XIC or an XIO doesn't KNOW or CARE why there is a "1" or why there is a "0" in the bit/box it is examining ... it just looks for the appropriate condtion - and then gives the appropriate evaluation ...

does that help? ... and thank you for the kind compliments on the scaling articles ...

 

[Martin T.]

What is still a bit foggy is the question of "show the state changes after 5 scans" In my mind after the first scan the state will be the same for the next 5 or 50 scans unless a command is given to change an input. This is what is giving me trouble.

Rung 0001 will change, it doesn't need a external command for C to become a 1 (true) statement. A and B in that rung are xio's.

Someone else will probably explain this better but I'll try.

With A and B xio's, look at it like a NC contact, flow will go through both until A or B's coil (OTE) are energized causing C to energize.

 

[goatom]

1's and 0's

Ron and all those on this thread:

All of your comments have been very helpful. Part of my block was considering a 1 as true and a 0 as false. Not true now that I have gone back and refreshed the basics. One writer in a course wrote
"Decide whether or not an input has been "met" ( ie if it is looking for a 1 and the circuit is sending a 1 to it, then it is "met") If all inputs in a rung are met then the output will turn on."

This rienforces what Ron was writing and cleared the fog. So now I can proceed with the orginal question with confidence. Not sure where I got off the rails, but I think I am back on. Thanks again to all. I am sure it is at times frustrating dealing with novice folks, but if we ( novices) did not have the help of you salts then life would be hard and my training station would probably go into a complete meltdown as opposed to smoking a bit from time to time.

AnkitJ I hope you are still following the comments. I did not mean to hijack the thread, but when I looked at the problem, I thought it was interesting that a basic question had me froze up.

Later I am sure

 

[sis]

Alternatives

Hi Goatom,

Sorry to interrupt. I tried different thing so called Smart Relay named Pico Soft 3.1 from Allen Bradley and that software is handling scanning procedure differently. I hope that I'm not mixing everything up what you have learnd so far but maybe worth looking at. I know that this Smart Relay is not a real PLC but it is a option for simple tasks. In this case only output C will take the "1" state and the reset on D is making that impossible that A can turn on "1". I plan to find out how the other equipment I have access to is going to handle this funny ladder logic. I find that there is a little purpose for using this ladder case else than only to find out how different PLCs scans this ladder case. I hope that you will get more practical things to learn from than something like that case.
Yours,
SIS.

 

[goatom]

Hey thanks a bunch. Any info is helpful. I appreciate your input. Have a good one. I think I am on the mend.

 

[mvandyke]

Don't let your eyes deceive you; your screen update isn't fast enough to see each scan.

 

[sis]

Single step

This is the outcome of a single step mode in Pico Soft 3.1.
There will be no blinking here because of the reset of D (RM4).
OK. I'm going to test the hardware, the Smart Relay.
SIS.

 

[sis]

The same result in the hardware.

C = "1"

Sis

 

[rsdoran]

Dilemma

Since sis brought it up, seeing the scan is not always necessary. I have 2 different SLC's, one fixed, other modular, they act differently to this code. Anyone have lab equipment that is capable of checking the differences? Then explaining why?

I went into this experiment expecting the image table to turn on A, yes D is latched then untlatches but I expected A to turn on because D latching should have told the image table A was 1, unlatching D would tell the image table D was 0, but not affect A.

I appear to get different results. I will see what happens using a GE Fanuc 90-30.

LOL, the student exercises are not as simple as thought!

 

[geniusintraining]

Ron I tried it on a DL06,

Ran it...got a head ache from the flashing on the screen , I think it was setting then the next scan reseting???

 

[Ron Beaufort]

I ran the code shown below on an SLC-5/04 ... I'm a little bit reluctant to show the step-by-step solution on the open forum ... I'd hate to thwart the educational process for untold future students ...

the main reason I’m posting is for the benefit of those of you who have RSLogix500 (sorry - not RSLogix5 or RSLogix5000) and a suitable processor to play with ... the figure below shows a VERY educational way to “single-step” through the logic and actually watch the show “rung-by-rung” ... the red cursor block will move down a notch every time another rung gets solved ... sort of takes the sport out of it I guess ...

 

[Ron Beaufort]

Greetings rsdoran ...

I have 2 different SLC's, one fixed, other modular, they act differently to this code. Anyone have lab equipment that is capable of checking the differences?

personally I'd have bet some serious money that both SLCs would work the same way ... I don't have a "fixed" to try this out with - but if I can possibly round one up from a friend, I'd love to see what causes any difference in operation ...

just wondering ... how did you keep track of "five scans" for each of the systems? ... did you use the "single-step" method that I mentioned above? ... if not, any difference between the fixed and the modular systems might be a result of simply "miscounting" how many scans had been processed in each one of the systems ...

 

[rsdoran]

I am looking into that, did it software and hardware, trying to see why different.