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Old April 27th, 2008, 08:00 AM   #1
PERSPOLIS
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Question STI (Selectable Timed Interrupts) function

HI can somebody explain STI preferably with example thanks a lot
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Old April 27th, 2008, 01:35 PM   #2
kamenges
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It's pretty much just what it says. The documentation for any of the processors that use the STI (PLC5, SLC5, MicroLogix) explains the STI pretty well. You may want to read through that and then post any specific questions you have.

Keith
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Old April 27th, 2008, 02:07 PM   #3
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Old April 27th, 2008, 02:28 PM   #4
Ron Beaufort
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here’s a quick explanation that has helped quite a few people ...



suppose that we have an operation in our program which MUST be done at a specific rate ... for example: we need to execute a PID instruction once each 100 milliseconds ...



now suppose that the regular scan time of our program fluctuates ... and sometimes it takes quite a bit more than 100 milliseconds between subsequent scans ... in this case, our PID operation might suffer ...



now suppose that we place the PID instruction in a new subroutine ... instead of using the customary JSR (Jump to Subroutine) instruction to send the processor over to execute the new file, we set the file up as an STI (Selectable Timed Interrupt) ... and we set the STI’s setpoint for the required 100 milliseconds ...



now suppose that we watch the processor scan through that LONNNNNG program of ours ... he starts down through the rungs - but as he goes along, he’s continuously keeping an eye on his little wristwatch ... as soon as the STI setpoint elapses, the processor stops what he’s doing in the regular program scan - and goes and executes the rungs contained in the STI file ... then he COMES BACK to where he was in the regular program rungs and picks back up right where he left off ... and the next time the STI setpoint elapses, the processor does the same thing: he leaves the regular program rungs - he goes and executes the STI rungs - he comes back to where he left off in the regular rungs ...


secret handshake: the STI tells the processor to INTERRUPT the regular scan at a TIME which we get to SELECT ... STI = ITS ...



that should be enough to give you the basic ideas ... but ... also consider that any FIELD INPUT and FIELD OUTPUT issues associated with the operations in the STI might also have to be taken care of within the STI ... that’s why you’ll often see rungs with “Immediate Input” and “Immediate Output” instructions included inside the STI file ...



you asked for an example ... I don’t usually post the exercises that I use in my classes - but if you want to call me up, I’ll walk you through one or two over the phone ... if you’ve got a spare system to play with during the call, it should only take about 10 or 15 minutes to nail everything down ...



hope this helps ...
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Last edited by Ron Beaufort; April 27th, 2008 at 02:31 PM.
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Old December 17th, 2013, 03:29 PM   #5
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Conversion

My question goes along with this but I have not been able to figure it out on my own. How do I convert an STI status function to RS5k? I am assuming I need to use some sort of GSV block but I have not been able to find the appropriate setup for the block for the STI function. Any help would be appreciated.
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Old December 17th, 2013, 03:44 PM   #6
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Nixalicious: On rslogix 5k use a perodic task.
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Old December 17th, 2013, 03:54 PM   #7
Ken Roach
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This is a dormant thread and you'll get better results if folks don't have to read the history to get to your question.

New Thread created, and question moved to here:

http://plctalk.net/qanda/showthread.php?p=570251
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