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what happen when OB1 scanning and Cyclic Interrupt OB come ?

Created by: RAWAT at: 10/26/2008 8:04 AM (7 Replies)

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10/26/2008 8:04 AM	Rate (1)
RAWAT Posts: 30 Rating: (1)	Dear sir If we usemore than one Cyclic Interrupt OBlike OB35 ,ob29 ,ob37.Now at time of OB1 scaning and this Cyclic Interrupt come at that time thenis OB1 stop the scanning ????? and Cyclic InterruptOB start ?????/ and after Cyclic Interrupt time OB1 further start scanning??? sir i dont understand what happen at that time when Cyclic Interrupt come ? or both(OB1and Cyclic Interrupt OB )work independentlypls tell with regards rawat
RAWAT Posts: 30 Rating: (1)	Last edited by: RAWAT at: 10/26/2008 8:18 AM Last edited by: RAWAT at: 10/26/2008 8:14 AM Last edited by: RAWAT at: 10/26/2008 8:11 AM Last edited by: RAWAT at: 10/26/2008 8:07 AM
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10/26/2008 8:57 AM	Rate (3)
Ayman Elotaify Platinum Expert Joined: 5/28/2008 Last visit: 10/29/2024 Posts: 4516 Rating: (839)	RAWAT Dear sir If we usemore than one Cyclic Interrupt OBlike OB35 ,ob29 ,ob37.Now at time of OB1 scaning and this Cyclic Interrupt come at that time thenis OB1 stop the scanning ????? and Cyclic InterruptOB start ?????/ and after Cyclic Interrupt time OB1 further start scanning??? sir i dont understand what happen at that time when Cyclic Interrupt come ? or both(OB1and Cyclic Interrupt OB )work independentlypls tell Dear RAWAT, A very good question. Normally when the CPU is in Run mode, OB1 is executing periodically. If an interrupt happen ( cyclic, time of day, hardware or error interrupt), The processor stop executing OB1at a break point, and starts to execute the interrupt and after finishing it will execute the OB1 from the breakoint. Here's attached a nice *.pdf file which explain in details what happen. If an interrupt happen and during this interrupt, another interrupt with a higher priority happen, then first,the processor will stop executing OB1 at a break point 1, and execute interrupt, Now another interrupt happen before finishing the first interrupt, the processor will stop executing the interrupt and goes the the second one ( with the higher priority) and after finishing, it will complete the first interrupt and after finishing it will complete executing OB1. Here's attached the pdf file Attachment PRO1_12e_OBs [Compatibility Mode].pdf (4515 Downloads)
	Ayman Elotaify
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10/27/2008 7:55 AM	Rate (0)
Ayman Elotaify Platinum Expert Joined: 5/28/2008 Last visit: 10/29/2024 Posts: 4516 Rating: (839)	RAWAT wrote in a privite message: sir If we usemore than one Cyclic Interrupt OBlike OB35 ,ob29 ,ob37.Now at time of OB1 scaning and this Cyclic Interrupt come at that time thenis OB1 stop the scanning ????? and Cyclic InterruptOB start ?????/ and after Cyclic Interrupt time OB1 further start scanning??? sir i dont understand what happen at that time when Cyclic Interrupt come ? or both(OB1and Cyclic Interrupt OB )work independentlypls tell with regards rawat Hello Rawat, If we use more than one interrupt OB, for example: OB35, OB29, OB37. OB1 is a cyclic execution organization block. While OB1 is executed and when the interrupt OB like Ob35 come, The Processor in the CPU will stop executing OB1. Or in other words, it will pause. Then it will starts executing OB35. After finishing OB35, It will continue executing OB1 from the point of pause. As I descriped in preavious thread as break point. But what will happen if more than one interrupt came in the same period? Here we have to think by priority of the OB. Always the highest priority OB will be executed first, then the lower one and so on. I hope my answer now is clear to you. With best regards,
	Last edited by: Ayman Elotaify at: 10/27/2008 7:57 AM Ayman Elotaify
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10/30/2008 10:07 AM	Rate (0)
RAWAT Posts: 30 Rating: (1)	Dear sir thanks for your reply i have one more doubt this is that if program in cyclic intrrupt is lengthy then this effect OB1 scanning or program ? with regards rawat
RAWAT Posts: 30 Rating: (1)
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10/30/2008 11:53 AM	Rate (0)
Aret Posts: 2348 Rating: (264)	Absolutely. Interrupt frequency and execution time both affect OB1 cycle time.
Aret Posts: 2348 Rating: (264)
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10/12/2012 8:48 AM	Rate (1)
doggalli Posts: 45 Rating: (1)	Dear Sir, I am Doggalli, Regarding this question, If Scan cycle time monitoring is 6000ms and In OB35 timer delay is 1 hour. How it will work, please tell me. Thanks and regards, Doggalli.
doggalli Posts: 45 Rating: (1)
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10/12/2012 12:33 PM	Rate (1)
dchartier Grandmaster-Expert Joined: 9/27/2006 Last visit: 11/2/2024 Posts: 12292 Rating: (2690)	Hello doggali; OB 35 is a cyclic OB, which means it will interrupt OB1 every time its cycle time is reached. In your example, OB35 would interrupt the OB1 main cycle OB every hour, for a few microseconds (just the time tolaunch the instructions porgrammed inside it). OB35 will not last an hour, if that is your question. In any case, anything that makes the OB1 scantime go over the maximum time allowed in CPU properties will call the watchdog timer. This is the ultimate protection of the PLC in case of programming errors, and will put the CPU to STOP mode. So too many interrupts (of any kind)from any OB might increase the scantime over the watchdog limit and trip the CPU. Hope this helps, Daniel Chartier

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8/25/2017 4:18 PM	Rate (1)
smiffy Platinum Expert Joined: 1/17/2007 Last visit: 10/21/2024 Posts: 1547 Rating: (537)	In a multi-tasking OS, the CPU can never ever execute "nothing". It always has to be executing something. In the S7 system you have a number of concurrent tasks (or threads) known as OB's. Some of these OB's are cyclic and some are linked to a specific events. The latter are more correctly referrred to as interupt OB's as they can interrupt the current process for them to be executed. This process is all controlled by what is known as a real-time scheduler. Each OB has its own priority. There is one "special" OB called OB1 (known as the "free cycle". This OB has the lowest priority in the system. The way the scheduler works is this: If there are no higher priority cyclic OB's to process and no higher priority interrupt OB's to process, then OB1 will be run untill it has completed, upon which it will be run again from the start. The time it takes to run OB1 will depend on the amount of code within it. This time may be further extended if higher priority OB's are run. When running ANY OB, if a higher priority OB wants to run (cycle time has ocurred or intterupt has ocurred), the current OB is "pre-empted" and it stops being run. The scheduler now stores all the runtime status data (data registers, address registers, status registers etc.) for the OB in a safe place (usually part of the OB local data). It then takes the saved data for the new OB that wants run, and restores it to the CPU, and then starts the CPU running the new task (OB), which will run from a run-time position that is last got to. The process is known as a "context-switch". When the higher priority OB has completed running it will be shelved until it one needs to run again. The scheduler will now run the next lowest priority OB that wants to run, until it once again gets to OB1. It is possible on some systems that OB's other than OB1 can be pre-empted by a higher priority OB, which themselves can be pre-empted by a higher priorty OB etc. Thus building up an interrupt stack of OB's wanting to run. The scheduler will then process the OB's in priority order until the list is cleared and OB1 is once again run. Thus you can see that the OB1 cycle time can vary somewhat based on the code within it and the interrupts that occur during its running.
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