One-Shot

Ok I am a little confused on one shots. Sorry for being a thick. In the section of learn PLC's on the main page I was going over One-shots. After the first scan when NO 1000 and NC 1001 complete the path. -DIFU- turns false after one scan. I understand that NO will return false and NC will return true. But what is turning NO 1001 true. to complete the path via NO 1001 and NC 1000.

Respectfully,
Miller

I think I understand where your confusion may lie. The output of a rung is not modified until all the conditions of the rung are evaluated. Until that time the output retains the state it had at the end of the previous scan. Thus, on this second scan, 1001 retains the ON state it had at the end of the previous scan and its NO contact in the bottom section of the rung is true. This completes the circuit while 1000 is OFF.

Would this make more sense:

   Signal                      OneShot

----| |------------------------[DIFU]-







   Signal     Temp             OneShot

----| |-------[OSR]-------------( }-







   Signal              Temp    OneShot

----| |------------+----|/|-----( )---

                   |

                   |            Temp

                   +------------( )---

First rung is typical for Japanese style PLCs.
Second rung is typical Allen Bradley style.
Third rung is "home made" one shot that should
work on just about any plc. Maybe this home made
version is what could make it easier to understand
how the "rising edge" is detected.

have fun

Good Job Panic......

I tried to get inside millerdtm's mind via his question. I think he understood the one-shot. But I think he was evaluating the output of the rung (from the one-shot lesson) after just interpreting the top branch of the parallel network then using that state of the output on the bottom branch. (Which is like a 90 degree rotated Modicon method of interpretation) Though maybe I was totally off base. If he ever answers back...

Panic mode told how to do it many ways.

Next is 'natural' one-shot "7217st4".

Panic Mode's demonstration

Panic Mode: That's a great illustration. I think I finally understand One-Shot's!

Bernie: Your description of how the scan works is very helpful, also. I guess I didn't understand that execution happens AFTER a scan.

Re: Panic Mode's demonstration

hickman said:

I guess I didn't understand that execution happens AFTER a scan.

Click to expand...

If you haven't already, also have a look at PHIL'S EXPLANATION...

beerchug

-Eric

I guess I didn't understand that execution happens AFTER a scan.

Click to expand...

Was it suppose to happen before the scan?

This logic stuff can be kind of confusing in ways.

I wonder if there is any relationship to relay logic involved?
Lets see..start pushbutton is pressed which activates control relay which in turn latches holding circuit for control relay.

Ever tried pushing the start button really fast? Try it.
Its dont always work..why? Because TIME is involved.

Even tho plc's work in micro or milliseconds there is always a time issue. The plc scans and looks at inputs then once scan is complete it updates outputs.

Electricity can have the same "time" issues on occasion. Using the above scenario if you press start pushbutton too fast then the momentary connection may not have "time" to latch/hold.

Time is always relevant to all things.

With plc's one of the first things to understand is the "SCAN" sequence and relevance to updating outputs.

Study Phils guide in more detail, it may offer more insight.

RE: after scan...

I didn't completely understand how a scan works; but after reading Phil's Explanation, my comment is true AND false. I understood that execution happens AFTER the PLC checks Input status. So, I was right. I did NOT understand that the PLC checks Output status AFTER the execution stage. So, I was wrong on that.

I guess I thought that the PLC checked all I/O on the same part of the scan.

I'm learnin'!

PLS DON'T SHOOT ME, I'M JUST A NEWBIE

Please tell me if I've been doing it wrong... My fellow mechanics used to power down the equipment when nothing seems to work right on the gas turbine logic. It is not a good practice but it was what worked for them. After attending Rockwell's PLC school I found it better to toggle the POWER-UP ONESHOT. It always worked, somehow the logic is properly reset. And oh, by the way, first time I did it, I forgot to stop the turbine first. No harm done, just redfaced, it was a spare turbine I kept offline from the rest of the facility just in case I screw up.

hickman, HERE'S a very good explanation (by Terry of course) of how the PLC scan operates. One of my favorites!...

beerchug

-Eric