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Old October 31st, 2008, 09:29 AM   #1
jimcav
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timer for scan time

i am using a micro logic 1500 ab plc i would like to setup a timer or counter to check scan time for the plc i am not sure that i trust s:35

jim cav
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Old October 31st, 2008, 09:38 AM   #2
Steve Bailey
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Set up a bit that alternates between on and off each scan. Then set up another bit to latch on upon command from you. Use the alternating bit to increment a counter with a preset of 500. Use the counter's DN bit to reset the latch. While the latch is true, enable a retentive timer. When the latch turns off, the timer's ACC value is the time for 1000 program scans. Divide by 1000 to get the average time for the 1000 scans.
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Old October 31st, 2008, 09:41 AM   #3
OZEE
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1) Why are you interested? (In 20 years of doing this, I've never seen scan time be an issue...)

2) Why don't you trust S:35?
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Old October 31st, 2008, 10:32 AM   #4
Brijm
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Quote:
Originally Posted by OZEE
1) Why are you interested? (In 20 years of doing this, I've never seen scan time be an issue...)

2) Why don't you trust S:35?
I had an older tech the one time, insist, that the scan time status bit was inaccurate on AB... This guy was very smart, but was also known to bend the truth a little, to make others look bad. I checked the scan time status, but he insisted that this was the problem (not his programmin of course). I was young at the time, and wanted to see for myself, as well as starting to catch on to how this guy operated.

So I set up a solid state output to toggle each scan, and put an oscilloscope on that output.... Turns out, that the status word matched the ocsilloscope exactly every time... The guy still said, that I was wrong, but the boss didn't seem to give it as much credit.

-MUR
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Old November 1st, 2008, 11:21 AM   #5
CeCo3
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I too connected an output to toggle once per scan, but instead of an oscilloscope, I connected a data acquisition instrument to it. (the advantage is I can review what I recorded.)

Anyhow, the scan time matched the S? bit (SLC5/04), but I discovered something interesting in the process. The scan rate was steady at 4msec, rock solid. However, when I went online with RSLogix500, every tenth scan or so would be 11msec. I attributed this to the the frequency at which RSLogix was being updating over the comm port.

For my application this was found to be a problem as it introduced variation into the load cell measurement being performed once per 50msec.

Congrats on using the oscope. It seems so few do. You've provided some degree of proof. Data driven decision making; your boss should have given you more credit, even on the side.

CeCo3
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Old November 2nd, 2008, 09:38 AM   #6
widelto
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Quote:
Originally Posted by CeCo3
For my application this was found to be a problem as it introduced variation into the load cell measurement being performed once per 50msec. CeCo3
Just to satisfy my curiosity, what is your applicaion so fast that you need 50 mS as an update time.
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Old November 2nd, 2008, 04:07 PM   #7
CeCo3
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One of my clients manufactures tablets using bi-layer rotary tablet presses similar to the ones pictured here:

http://www.aaronequipment.com/FullSi.../520594a.photo
http://www.aaronequipment.com/FullSi...s/86932a.photo

The presses spit out finished tablets at a rate of more than 1,150/minute (1/(1150/60)=52.2msec). A load cell at each layer measures the amount of registered force when the granulation if compressed a fixed amount. Precise timing is needed to ensure consistent measurement is performed cycle after cycle. As I mentioned previously, when RSLogix was connected to the PLC, every tenth scan or so was delayed an additional 7msec and this caused a lower force value to be registered. It took some serious investigation to identify the cause of that problem.

Back to the original issue of the S35 time being accurate, I've never had reason to doubt the accuracy of the average scan reported in any A-B PLCs. But after what I've learned about scans, the only way your going to know for certain is to monitor the S35 bit (or equivalent) by tieing it to an output channel and monitoring it with some sort of recording instrument.

There are other things to consider as well, such as event-based subroutines that do not execute every scan.

I hope this answered your question.

CeCo3
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