On Delay vs. Off Delay, timing

kowen

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Mar 2011
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Peoria, IL
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I am new to ladder logic and am using a Koyo plc with Click software. There doesn't seem to be a step-by-step execution simulator for this software to practice with, so trying to learn by running the actual plc. I often need to use a timer for device settings that come on and off, and even to change rpm's etc. But I seem to run into issues with the timers, and with changing coil statuses within a scan cycle. Any tips for use of on-delays vs. off-delays for timer control? Any tips for keeping coil statuses clean and independent of cycle times?
 
I'll try to respond because no one else did.

I am new to ladder logic and am using a Koyo plc with Click software. There doesn't seem to be a step-by-step execution simulator for this software to practice with, so trying to learn by running the actual plc.

Many of us prefer to experiment with code on an actual PLC.


Any tips for keeping coil statuses clean and independent of cycle times?

I recommend visiting Ron Beaufort's site for PLC training. And viewing the videos (P.S. view them in order). They are for another brand of PLC but the concepts are the same.
http://www.ronbeaufort.com/

Here is another post by Ron concerning Automation Direct PLC's and their unique use of retentive ranges.
http://www.plctalk.net/qanda/showthread.php?p=71786&postcount=1

And finally a little code to demonstrate how changing the position of a rung can change the outcome.
 
Vaughn - thanks for your very helpful reply. I will go through Ron's videos right away. And, thanks for the example code ... I assume Rung 3 sets C2, but Rung 1 does not, since crossed over into a new scan cycle. Yes ... this is the kind of thing I am trying to get a better feel for. In addition, the concept of resolving net states at the end of a cycle, vs logic states for each rung, one at a time.
Thanks to all the "teachers" that help us learn.
Ken
 
Regarding timers - the Done/Finished contact state of a timer (TON) is typically not set until the point in the execution of the scan where the timer instruction is encountered and the current time value is equal to or greater than the preset. Obviously then a timer is accurate to 1 scan time.

Timed interrupts may be more accurate in terms of absolute time but then you may have actions occurring in the middle of a scan. Sometimes it is best to ask yourself - exactly how accurate do I need this timing to be?

Long term timing may best be served by utilizing the system's Real Time Clock, if available. Very short timings, as mentioned before, may be best executed by interrupt coupled with 'immediate' input or output instructions. But there is an execution time hit to be taken into account. It's never really easy when approaching the envelope of the PLC's capabilities.

An example of the envelope conditions not Ron's warnings in the posting which follows.
 
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I can do most everything using the timer on instruction, (timer enable, timer timing, and timer done bits). The two types of timers act a little different, and I have found it to be a little confusing using the timer off instruction.
 
be very careful when trying to "adapt" a TON timer to do a TOF timer's job ... the system might act PERFECTLY for years in normal day-to-day operation - but then suddenly give a dangerous "surprise" after a power cycle or in other "go-to-run" situations ...

for a quick example, check out Figure A and the associated questions here:

http://www.ronbeaufort.com/QUIZ_121_QUESTIONS.pdf
 

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