S7 Scan times

CharlesM

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C
Join Date
Aug 2005
Location
Arkansas
Posts
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What kind of scan times can I expect on a Siemens S7 300 system with a 315-318 processor?

I am using WinAC RTX (soft PLC) and im getting 10ms. My local distributor thinks a regular plc could be faster.
 
What scan time do you need ? 10ms sounds fine.

A 318 was approx 10 times quicker than a 315 when I tried to use one 18 months back. FC's with UDT parameters (from my library of function) meant the 315 wouldn't execute the 10ms interrupt stuff quick enough so it was more cost effective to use the 318 rather than re-code the FC's to run quicker.
 
I am using a little process controller doing a function at 1-2ms. My local rep said we could do it with a plc. I have never seen a plc that fast.
 
You can set the timed interrupt (OB35) to run at 1ms so the scan time is then not important. You just need to be able to do your time critical stuff inside 1ms so what sort of processing needs to be done every 1ms and what will it be controlling ?
 
It depends on what you have configured. I once used it with some middleware that checked changes in DB's and a few robots and I got up to 500 ms in heavy traffic. Most likely you are not going to pass 10ms.
 
I use an S7 318-2DP in a project I'm working on. This project is by far the largests I've done in my carreer. Still the cycle time was around 8ms. (Which surprised me) On a 314 I normally have 8-13ms with programs that are much less demanding. So I think the 318 is a good option. But maybe you're application is larger/more demanding?
 
I will be looking for the peak of an analog signal and turn on an output to stop the machine if it go's over a set point. The program would be small but I guess I need to look at the speed of the I/O modules as well. Thanks for all the suggestions.
 

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