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Created by: adilmahsan at: 7/21/2009 11:03 AM (4 Replies)

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7/21/2009 11:03 AM	Rate (0)
adilmahsan Posts: 71 Rating: (2)	I have a question regarding timers in step 7 I had made a program with a scan cycle of 20 msec... The program had a pulse generator using two timers... The problem was that when i had set the timer value to anything less than 30 msec (20 or 10 msec), the generator would work for a cycle or two and then stop Could anyone please tell me how exactly do these timers work??? Are they polled or interrupt timers?? Thanks
adilmahsan Posts: 71 Rating: (2)
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7/21/2009 2:23 PM	Rate (1)
Aret Posts: 2348 Rating: (264)	Guess your problem is that timers dont get 0->1 change, they get 1->1->1 so they do not restart. Alternitevly you can: Use SFC64 to find out how much time passed Use SFC1 - same purpose Use some OB3x (35 for example) and replace times with counters Everything depends on what exactly are you trying to do with such low timer's values.
Aret Posts: 2348 Rating: (264)
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7/21/2009 3:15 PM	Rate (1)
fritz Diamond Expert Joined: 10/7/2005 Last visit: 10/4/2024 Posts: 3024 Rating: (1054)	adilmahsan Could anyone please tell me how exactly do these timers work??? Hello adilmahsan it depends on the Timers you are using so let's start with the good old so called S5Timers (e.g T1, T2 , T3 etc.). S5 Timers: These are part of the Operating system of your S7 and are updated by the OSin the "background" as per their "Time base" (or resolution). For example, if your "Time Base" is 10 ms, then your S5 Timer value is updated every10ms. More importantly it will "time out" at any time in the scan when the time isup. As for the elusive"Time Base", this is another thing worthwhile noting about S5 Timers, especially since (unlike in an S5 where these "come from") it is sort of hidden from you in S7 due to usage of the more user friendly S5TIME format. The"Time base" that is automatically used by the OSis as listed below and depends on the choosen Preset Time. Note thatit is valid for the WHOLE Time that the Timer is running!: Time base: 0.01 sec. for a range of 10ms to 9.99 sec. Time base: 0.1 sec. for a range of 10sec. to 99.9 sec. (1 min. 39sec. 900ms) Time base: 1 sec. for a range of 100sec. to 999 sec. (16 min. 39sec.) Time base: 10sec. for a range of 1000sec. to 9990 sec. (2 h 46 min. 30sec.) Attached is a picture which hopefully makes this a bit clearer. What does this all mean for your "problem"?. Well, assuming you do NOT use the R(eset) input of your S5 Timer, you'll need a 0-->1 transition to start your Timer. It really depends on how you wrote your program but assuming you use the S5 Timer to retrigger itself (e.g AN T 7/ SD T7), your scan of the S5 Timer will "miss" the "0" call of the S5 Timer(since it expired after the inital start and beforeyour scan called it the second time)and it won't run again. IECTimers: The other Timers that are available in an S7 are the IEC Timers (i.e. SFB 3, 4 & 5) and can be found in the System Functions of the Standard Library. These are NOT updated by the OS, but instead each timethey are called and do NOT have a "variable" Time Base (their resolution is 10ms in an S7-300 and 1 ms in an S7-400). I hope this helps Attachment S5 Timer Time base.jpg (421 Downloads)
	Last edited by: fritz at: 7/21/2009 3:19 PM Forgot the attachmnet and noted that Aret had already beaten me to this post (sorry mate, dind''''t mean to copy you, I guess I have to learn to type faster or less or both...) Cheers fritz The advice and opinion given in this thread is that of the author and does not necessarily reflect the views of I S Systems Pty Limited. To find out how I S Systems can help you with your automation needs please visit www.issystems.com.au.
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7/22/2009 5:33 AM	Rate (1)
adilmahsan Posts: 71 Rating: (2)	thanks for the replies here's how i did it... i used 2 timers.. the negative edge of the first triggered the 2nd and the negative edge of the 2nd, triggered the 1st... now, this program worked well for timer values equal to or greater than 30 seconds but not for 20 msec and 10 msec
adilmahsan Posts: 71 Rating: (2)
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7/22/2009 7:23 AM	Rate (1)
Aret Posts: 2348 Rating: (264)	Absolutely right - and you can't ged rid of it. So what you can do: Asume, that we must set bitfor 30ms, then reset it for 20ms. We can make interrupt OB with time 10ms with such code L "some_sounter" +1 MOD 5 T "some_sounter" L 2 // 0,1,2 = set <=I = "Output_bit" But again - everything depends on what functionality are you trying to achieve. Ps. Fritz, it's not about typing speed - your post is just LARGER))))))))))))))
Aret Posts: 2348 Rating: (264)
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