# PLC Logic Scan

Discussion in 'Homework Help' started by Aviku, May 23, 2009.

1. ### Aviku Thread Starter Member

Nov 20, 2007
11
0
I know that there are two scan methods can be used for scanning for logic solving. The rungs of ladder diagram are solved from left to right and top to bottom or scanning a page from top left to bottom and then up again.

What I dont understand which method holds the upperhand in terms of fast switching.

Appreciate if someone could explain

2. ### Aviku Thread Starter Member

Nov 20, 2007
11
0
Is there anyone who could help me to clear up my mind?

I'm bit confused of understanding the concept.

3. ### Papabravo Expert

Feb 24, 2006
12,276
2,722
Perhaps you could give us some additional context for "fast switching".

AFAIK the speed of solving a ladder is a function o the number of rungs and the complexity of the function blocks. You were not exactly clear on the second method. In fact I did not think there wa smore than one way.

4. ### Aviku Thread Starter Member

Nov 20, 2007
11
0
OK, I have attached the document which contains explanation from the book.

As you said, I couldn't find anywhere both methods and comparision

The part in BLUE in the document I don't understand

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5. ### gryskop Member

Mar 1, 2008
26
0
Is this what you are looking for?

Also, check out this Google search string

6. ### Aviku Thread Starter Member

Nov 20, 2007
11
0
Actually question is about PLC Logic solving. In logic solving we use logic solving sequence or scan. As we can see the scanning can be from left to right or top to bottom (please see in the attachment).

OK, one method should be better than the other. What I don't understand is which method holds the upper hand and against what?

I know this is tricky and that's why I'm also lost in this.

7. ### Papabravo Expert

Feb 24, 2006
12,276
2,722
I'm quite sure that it is not obvious which method is more vulnerable to a rapidly changing input. Shannon's well known sampling theorem says that you must sample a system at twice the frequency of the highest frequency component in order to capture the behavior of the system. As I understand PLC operation, all of the inputs in the input image table are frozen at a particular instant in time. That is, no updates are performed during the solving of the logic. Then all the out updates are performed at one time. This suggests that PLC designers might choose the top to bottom left to right method.

I think the book is refering to an illusory problem. If the scan rate is 5 milliseconds then the fastest input that would be reasonable would be 100 Hz. In my experience there are very few PLC inputs that run in excess of 100 Hz, uless someone is trying to close a velocity loop on a servo drive. For discrete I/O points this is normally more than adequate.