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Old July 25th, 2007, 02:19 AM   #1
PLC_talk
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SLC 500 PID problem

Hello ALL,
I am facing a problem while executing the PID instruction. As soon as the Processor is put in Run mode the CPU goes to fault indicating PID fault. If the instruction is deleted then the rest of the program works fine.
I have set the parameters as follows -
Kc-0.5, Ti- 0.1, Td- 0.0 , Loop update - 0.2 , Mode- Sp-PV, Control - Auto, Time mode - STI, SP MAx- 16383, SP min 0, CV Max - 100, CV MIn -0
In the processor status-STI tab S30= 20, S31 = 7, s:2/1 = 1

Is the S31 related to the Program file where the PID instruction is placed? Mine is in File 7.
In Run mode my Loop update time gets reset to 0.

I am not able to find the exact cause of this, so would appreciate any pointers in right direction.

Thanks,
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Old July 25th, 2007, 04:20 AM   #2
Myles
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Change the timed mode to TIMED rather than STI. I had this problem before and that sorted it.
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Old July 25th, 2007, 12:41 PM   #3
Ron Beaufort
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Greetings PLC talk ...



Quote:
In Run mode my Loop update time gets reset to 0.


based on this statement, I’d bet (more than pocket change) that you have a “memory overlap” situation ... specifically, something else beside your PID might also be using part of the PID’s 23-word memory block ... that “something else” could be another ladder logic instruction - or it might be an HMI/SCADA station that’s writing into that memory area ...







in the example above, the 23 words starting at N10:0 are dedicated to the PID ... so N10:0 through (and including) N10:22 should not be “written into” except very carefully to change the setpoint, etc. ...
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Old July 26th, 2007, 12:28 AM   #4
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Thanks Ron / Myles,
I will reassign some other N file & test it. BTW the error I am getting is " An invalid value is being used for the PID instruction parameter" . Also in the RUN mode the MODE gets set to TIMED & loop update to 0.

Regards,
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Old July 26th, 2007, 09:53 AM   #5
Mickey
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Post your code (.RSS file)zip it then post it. If you can.
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Old July 26th, 2007, 10:08 AM   #6
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What is the value used for the PV scaled to? The PV must be scaled to a value between 0 and 16383; anything above/below this will cause that type of fault. If it is scaled to that value, see if there is any logic in place to prevent under/over-ranging the PV address. Even if scaled properly (0-16383), if the device providing the PV value (transmitter, etc.) fails, you could get a high or low out of range value.
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Old July 29th, 2007, 11:53 PM   #7
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Hello,
Thanks for all the inputs.I was able to solve the problem by setting the loop scan to 1.2.
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Old July 30th, 2007, 11:06 AM   #8
Ron Beaufort
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Greetings PLC_talk ...



we’re glad that you’ve found a solution - and if you’re happy, then we’re happy ...



but ...



there has to be more to it than just setting your “loop scan” (Loop Update?) to 1.2 ... in your first post you mentioned that you were setting the Loop Update to 0.2 ... and then you said:



Quote:
In Run mode my Loop update time gets reset to 0.




so the question remains: if the original setting of 0.2 was being reset to 0, then why is the new 1.2 setting not being reset to 0 also? ...



but if it’s working and you’re satisfied, then I suppose that’s a happy ending ... still, I’ve got a hunch that you did a little bit more to get this system working than simply changing the Loop Update setting from 0.2 to 1.2 ...



anyway ... good luck with the rest of your project ...
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