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IEC Timer

Created by: oldfalg at: 4/13/2009 5:54 PM (3 Replies)

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4/13/2009 5:54 PM	Rate (0)
oldfalg Posts: 28 Rating: (1)	Dear friend: I have a question about IEC Timer. we use internal timer oftern in CPUfor example T1/T2,they are measured by hardare clock,but the count is limited. IEC timer (SFB3),it's a system function block and we can use it more then the count limited.but IEC timer is measure by software in CPU, there is an imprecision. I want to know the influence factors of precision,may be the different OB block or........ and can you tell me the number of deviation. Thanks for your support.
oldfalg Posts: 28 Rating: (1)
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4/14/2009 11:47 AM	Rate (0)
Denilson Pegaia Platinum Member Joined: 9/23/2005 Last visit: 10/10/2024 Posts: 4349 Rating: (1446)	Hi, Genaeraly, the software based timing read internaly the clock of the CPU. So I don't find that there is differences between SFB4 and T44 precision, for example. Take also a look at the faq http://support.automation.siemens.com/WW/view/en/26116927
	Denilson Pegaia Siemens Industry Sector Technical Support
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4/15/2009 4:28 AM	Rate (0)
oldfalg Posts: 28 Rating: (1)	Hi: I found a discription in S7_distributed_safety manual, below: When using an F-application block with time processing, take the following timing imprecision sources into account when determining your response times: • Known timing imprecision (based on standard systems) resulting from cyclic processing • Timing imprecision resulting from the update time of the time base used in the F-application block (see figure in "F-Application Blocks") • Tolerance of internal time monitoring in the F-CPU – For time values up to 100 ms, a maximum of 20% of the (configured) time value – For time values starting at 100 ms, a maximum of 2% of the (configured) time value for example FB184,this functionality is based on IEC TIMER SFB 3 "TP"). I don't understand the meaning of described above. thank for your help
oldfalg Posts: 28 Rating: (1)
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4/15/2009 5:21 PM	Rate (0)
Denilson Pegaia Platinum Member Joined: 9/23/2005 Last visit: 10/10/2024 Posts: 4349 Rating: (1446)	oldfalg Hi: I found a discription in S7_distributed_safety manual, below: When using an F-application block with time processing, take the following timing imprecision sources into account when determining your response times: • Known timing imprecision (based on standard systems) resulting from cyclic processing • Timing imprecision resulting from the update time of the time base used in the F-application block (see figure in "F-Application Blocks") • Tolerance of internal time monitoring in the F-CPU – For time values up to 100 ms, a maximum of 20% of the (configured) time value – For time values starting at 100 ms, a maximum of 2% of the (configured) time value for example FB184,this functionality is based on IEC TIMER SFB 3 "TP"). I don't understand the meaning of described above. thank for your help Hi, Response time is very important in fail safe application, also as safety reasons. In this context, if you use a timer for a safe propouse, you must know, that the preset time must differ of the action time for the above mentionet reasons. It's nota question about measurement precision, but of response time. In this context, the use of a Time or a SFB for "timing" has no differences.
	Denilson Pegaia Siemens Industry Sector Technical Support
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