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Question about cyclic interrupt OB’s

Created by: zwobbel at: 12/11/2009 4:45 PM (7 Replies)

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12/11/2009 4:45 PM	Rate (0)
zwobbel Posts: 3 Rating: (4)	Hello all, I have a question about cyclic interrupt OB’s. (PCS7 - CPU417-4 H). What happens if the code executed in the cyclic interrupted OB35 that is interrupting every 500ms takes 600ms for only “1” cycle? (Normal code execution time in OB35 is 100ms) Let’s say it happens once every hour? Is this OB35 interrupted after 500ms by the same OB35? Or is the next OB35 interrupt not executed? Or does the next OB35 interrupt start after 600ms? Thanks in advance!
zwobbel Posts: 3 Rating: (4)
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12/13/2009 10:05 PM	Rate (0)
Yer Posts: 86 Rating: (7)	Hi, By me best kown. -OB 35 interrupta next time after 500 ms. -the remaining 100 ms is not executed. -other OB cycles cross the OB 35. this will cost extra cycletime. it is possible to visualise the cycle time. You also can tune the ob cyclus for example by using the offset. Yer
Yer Posts: 86 Rating: (7)
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12/14/2009 2:23 AM	Rate (0)
Ayman Elotaify Platinum Expert Joined: 5/28/2008 Last visit: 10/7/2024 Posts: 4515 Rating: (839)	Yer is correct. I would like to explain something here. As after the CPU make a start up and begin executing OB1, why the CPU is being interrupted to execute othe OBs? Thus because the parameter called "Priority". Each Interrupt OB, have a priority number or index. So, generally, if the trigger condition of the OB happen while the CPU is executing other lower priority OB, it will be interrupted at a break poit and execute the higher priority OB and then return to the lower priority OB at the break point to continue execution and then return to continue OB1. I would like to attach a file here which descripe how the CPUs execute the programs in the different OBs. Attachment PRO1_12e_OBs [Compatibility Mode].pdf (1207 Downloads)
	Ayman Elotaify
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12/14/2009 7:06 AM	Rate (0)
Yoga Shankar Posts: 288 Rating: (22)	Dear All, I have a slightly different opinion about the question we are discussing here. All Siemens PLC CPU's have a hardware watchdog timer which will trigger a system fault if the any OB is interrupted by itself before completion of the entire code in that OB. Hence, if the code in an OB takes 600 ms to execute and the cylic interrupt time of the OB itself is less than that , say 500 ms then the CPU will go into a system fault. I may be wrong also. If so, please correct any mistakes here. Regards, Yoga
Yoga Shankar Posts: 288 Rating: (22)	Last edited by: Yoga Shankar at: 12/14/2009 7:07 AM
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12/14/2009 9:07 AM	Rate (1)
Yer Posts: 86 Rating: (7)	Dear all, Basic it is right that there is a system fault. But I believe there is a fault-OB called (such as OB 80) Option 1: when the OB is not loaded in the CPU the CPU go to Stop mode Option 2: But if the OB is in the CPU the CPU runs trough the OB and stay in Run mode. Yer
Yer Posts: 86 Rating: (7)
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12/14/2009 9:24 AM	Rate (6)
daga Gold Member Joined: 3/18/2009 Last visit: 5/6/2024 Posts: 306 Rating: (94)	zwobbel Hello all, I have a question about cyclic interrupt OB’s. (PCS7 - CPU417-4 H). What happens if the code executed in the cyclic interrupted OB35 that is interrupting every 500ms takes 600ms for only “1” cycle? (Normal code execution time in OB35 is 100ms) Let’s say it happens once every hour? Is this OB35 interrupted after 500ms by the same OB35? No. OB35 cannnot be interrupted again by OB35. They both have same priority. (Ayman's and Yer's explanation holds good if you have an OB with higher priority interrupting the current OB. But in case of same priority interrupt (OB35 trying to interrupt OB35) I expect the behaviour I describe below zwobbel Or is the next OB35 interrupt not executed? Not exactly. May be in some cases. I will try and explain it. zwobbel Or does the next OB35 interrupt start after 600ms? Yes, more or less, this is what really happens. However, a little trickier. When the OB35 code takes 600ms for 1 cycle, another OB35 trigger is received before the previous OB35 execution is completed. This causes an OB80 trigger. If OB80 is not loaded in the system, then CPU goes to stop. If OB80 is present in CPU, then The new call of the OB35 is buffered. (start information is buffered) OB80 is executed. Control returns to the previously running OB35, which then executes the remaining 100ms of its code. Immediately after this another call of OB35 is executed (with the buffered start information) Now, assuming that your code goes to 1100ms for 1 scan in an hour, then two OB 35 calls will be buffered and executed immediately after the current OB35 scan is completed. However the number of calls that can be buffered is limited. (I am not sure what decides thesize of "start information buffer") So even though your code was executed after 600ms (instead of 500), due to buffering of start information, all timers and codes which are using the OB start information see the execution to be after 500ms. (for example, if you measure time difference between the two OB starts using the TEMP variable OB3x_Date_Time, you will always see it to bee 500ms and never 600ms) Since you are using PCS7, and I am assuming that you have the latest CPU 417 (supporting SFC78) then the above mentioned situation is handled by a block called as CPU_RT (FB_128). Reading the help of this block will also help.

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12/14/2009 10:32 AM	Rate (0)
zwobbel Posts: 3 Rating: (4)	Hi All,Thanks for all the help in answering my questions ! Especially “daga” he gave me the exact answers that I was looking for ! Thanks again! Regards, Carlos (Zwobbel)
zwobbel Posts: 3 Rating: (4)	Last edited by: zwobbel at: 12/14/2009 10:32 AM
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10/10/2019 12:27 PM	Rate (0)
Piotr.M Platinum Expert Joined: 6/24/2013 Last visit: 10/7/2024 Posts: 1628 Rating: (421)	Hi Also beware of the following - when OB35 is set to a very short period i.e. 10ms monitoring of that OB will extend its cycle time enough to trigger PLC STOP!

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