practise assignments and a stop/start problem

rwhite · Mar 21, 2012

Hi guys

I did a few PLC courses in electrical college but the 2nd was quite poorly run.
Getting back into the work of programming PLC's as was just wondering if anyone here knew of any resources for practise assignments?

Just some cool projects I could work on and gain experience.

one more question....
At college one of the problem questions we had was to make a single push button turn a relay on and off.
no timers, OSR etc...just straight ladder logic

One of the guys if I recall correctly did it in about 3 lines of logic....

I did search the forums but didn't find an exact solution..I saw alot of people getting called out for not searching first, so sorry but the answers didn't apply to this problem
(at least the ones I found)

leitmotif · Mar 21, 2012

Look up flip flop. Should get more info than you know what to do with.

Dan Bentler

Lancie1 · Mar 21, 2012

... was just wondering if anyone here knew of any resources for practise assignments?

Yes, hundreds have been posted here over the years. So many, that to help you, we would need to know what type of program you are interested in, what brand of PLC, and what model (PLCs are not generic, but each has its own software and program instructions).

Look in the "Downloads" section for many student problems. Click on the yellow "Downloads" at the top of your screen. I have several student problems that I have collected over the years. Just say what you need, and where you want to start.

One of the guys if I recall correctly did it in about 3 lines of logic....

Click to expand...

Alternator logic can be done on one rung of PLC logic, but the complexity of the logic depends on what instructions are available on the specific PLC that you are using. Some are cheap stripped-down PLCs, so need more instructions and more rungs to do the same job.

Here are 10 methods to do PLC logic that alternates an output on and off for each pushbutton press.

rwhite · Mar 22, 2012

Thanks so much!

I am using the free version of rslogix with the simulator program so no PLC to speak of. So all I can do is toggle stuff with the program to simulate (speaking of is there a shortcut/hot key to toggle devices when selected?)

Now with the flip flop, maybe I mean something else...

The exercise was to have one push button that would turn an output on (and hold it in) and then off with each press of the input

I tried the first alternator example and it seems the internal relay comes on first , then the output with it, then the internal relay turns off while the output remains one and then finally the output turns off

Maybe I have dones something wrong?

rwhite · Mar 23, 2012

whoops it is working fine

Does anyone know of a hotkey to toggle inputs rather than right click/toggle input?

would be nice and alot quicker

MarkNightingale · Mar 23, 2012

Ctrl+T

Mark

rwhite · Mar 23, 2012

MarkNightingale said:
Ctrl+T

Mark

Hi mark

I'm using rslogix micro starter light
If I click on an input (or go the input bit page) ctrl-t doesn't do anything (only way I can make it work is right click/toggle or change it from a 0 to a 1.)

Just a question about 'which rung wins'
I uploaded one of the alternator examples and I'm having trouble with B3:0/9 and B3:0/8 on the first rung

looking at the logic I would have thought B3:0/8 relay becoming true would interupt the contact before B3:0/9 could become true.

But testing the program it is not the case.
How does this work?

Eric Nelson · Mar 23, 2012

A-B scans left to right, top to bottom. Therefore, it scans the XIO B3:0/8 contact FIRST, which is still TRUE at that point.

-Eric

rwhite · Mar 23, 2012

Ok so is this correct?

I:1/0 true, XIC B3:0/8 true, so B3:0/9 output is true
I:1/0 true so B3:0/8 is true

jump to next line

B3:0/9 XIO true , XIC O:2/6 true , O:2/6 true
B3:0/9 XIC false, XIO O:2/6 true ,O:2/6 false(?)

I'm lost

rwhite · Mar 23, 2012

sorry I can't edit the post I messed up the XIC/XIO's

Eric Nelson · Mar 23, 2012

Might help to read Phil's tutorial, and be sure to watch the animation.

http://www.plcs.net/chapters/oneshot18.htm

-Eric

daba · Mar 23, 2012

Lancie1 said:
Here are 10 methods to do PLC logic that alternates an output on and off for each pushbutton press.

Numbers 2, 3, and 10 are the only ones that can be considered a true "flip/flop" or "bistable" circuit.

All of the others may suffer memory loss at power-on or change to run mode. This is certainly true for A-B PLCs, and possibly others. In that sense, they do not meet the specification of changing state "for each pushbutton press".

Many people forget to assess all the possibilities. Of course, you have to decide whether the flip/flop or toggle you are programming should be retentive.....

Lancie1 · Apr 22, 2012

Daba,

I never said they were flip-flops. That is other people's misguided incorrect misnomer that tries to pretend that this PLC logic performs the same function as the electronic device called a flip-flop. It does not, and the use of "flip-flop" to describe the PLC version should be discontinued. I certainly never call it that. That is why I use the term "alternator" to better describe PLC logic that works while the PLC is in RUN mode. When not in RUN mode, I do not care what happens to the alternator, although given a choice it is to have the alternator always be OFF when going from program mode to RUN mode.

daba · Apr 23, 2012

Lancie1 said:
Daba,

I never said they were flip-flops. That is other people's misguided incorrect misnomer that tries to pretend that this PLC logic performs the same function as the electronic device called a flip-flop. It does not, and the use of "flip-flop" to describe the PLC version should be discontinued. I certainly never call it that. That is why I use the term "alternator" to better describe PLC logic that works while the PLC is in RUN mode. When not in RUN mode, I do not care what happens to the alternator, although given a choice it is to have the alternator always be OFF when going from program mode to RUN mode.

But in control systems that use "alternators", they certainly must not be reset by the PLC starting up again.

The alternator, bistable, flip-flop, or whatever it is called, should not be changed by the system, only by the logic behind it.

Memories that get reset by the PLC power-up cycle could have severe consequences in many applications.

I favour the opposite to your viewpoint - I want all of my "alternators" to remember the state they were in at all times, even through a power-cycle. If I choose to do so, I can always program a reset of specific alternators using a "power-up", or "first-scan" bit. In that way the user program has complete control of the alternator state.

Lancie1 · Apr 26, 2012

I favour the opposite to your viewpoint - I want all of my "alternators" to remember the state they were in at all times, even through a power-cycle. If I choose to do so, I can always program a reset of specific alternators using a "power-up", or "first-scan" bit. In that way the user program has complete control of the alternator state.

That is great for the experts, but many that come here are beginners. I see too many motor circuits that have the potential to restart a motor after a power failure. If that motor happens to be running a crusher, grinder, mixer, cutter, punch press, or other dangerous machine, the potential for hurt greatly outweighs any other concern. The last Joe I heard about was sent down to "clean out the drum while we are down due to the power outage. Unknown to Joe, the machine had been rewired so that the start circuit stayed on through power failures. The previous shift had been running the machine when the power failed. Joe figured he would save time and not bother with the lockout (no one else was within a half-mile of the drum). He crawled in, the power came back and Joe was spit out the other end in little pieces. The sad thing was that Joe did not even make good aggregate - most of the little bloody pieces of Joe wound up in the REJECT pile.

I hope that NONE of the alternator methods that I posted can or will be used to keep a motor running through a power failure.

practise assignments and a stop/start problem

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