How to set cycle time in Step 7?

S7PP

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Join Date
Sep 2010
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Toronto
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Hi experts, I'm wondering whether I can set the scan cycle time in step 7, for example, how to set it to 50ms if I want the program (OB1) is scaned once per 50ms?
 
Last edited:
you mean it's not able to set the scan time for OB1?

yes, I may use OB35 for 100ms per scan, or so.. . So you think that's the only way?
 
S7PP said:
you mean it's not able to set the scan time for OB1?

yes, I may use OB35 for 100ms per scan, or so.. . So you think that's the only way?
Click to expand...

You can't to my knowledge set scan time directly, but as mentioned above you can use one of the OB's to call a block repetitively at the required scan time (for example, a PID control block).
 
In the top of OB1 you can put a self-triggering timer, something like this: NB! The time will allways be 50ms+.
A M 10.0
L S5T#50ms
SD T 10
AN T 10
= M 10.0
BEC

L MW12 // count just to check if the code works
+ 1
T MW12

Kalle
 
You haven't stated which PLC.
The 400 series have the option to set a minimum scan cycle time. The 300's don't.
For the 300's you need to create the interval yourself.
 
OB1 is for free running cycle.
OB35 is for exact timed cyclic interrupt.

Use OB35 if you want exact cycle time.
Depending on the CPU type, there may be additional OBs for more cyclic interrupts available, so you can have 50 ms, 200 ms, 500 ms cycles running in parralel.
 
With S7-400 you can set cycle time minimum. With S7-300 only way to do constant cycle time is by using above mentioned interrupt OBs.
 

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