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PLC Scan Time Clarification

7/28/2009 2:38 AM	Rate (0)
Maha Posts: 118 Rating: (4)	Hi All, we are using CPU 414-4H System forone Application, We started testing our Project in our office without any Field IO's. (only CPU and one Input module on IM153-2) After we downloded our project in to the CPU, we checked the Scan time it is 60ms. we got shocked, because for our application we must have scantime within 25ms. we tried to check our logic in all the ways and reduced all the loops used in the program..still not much change in the scantime. Our IO range is 0 to 700 bytes After lot of way of trying, finalywe tried to change our process image input/output area to 800 bytes (initaly it was 256 default value). then the scan time was reduced to 7 ms.....now its very nice. So what may be the reason for this type of changes? How its make the sense?
Maha Posts: 118 Rating: (4)
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7/28/2009 5:12 AM	Rate (0)
H-H Platinum Member Joined: 7/2/2008 Last visit: 8/7/2025 Posts: 928 Rating: (309)	Hello Maha, Sounds strange but are you sure that only changing of process image size was the key? If you had process image of 256 bytes and for instance you write L IW512 then you will have internal fault (Programming error) you are trying to access wrong address. So this will cause an interrupt (OB121) at each wrong address (Addresses above 256)and will increase scan cycle for sure. The second way, maybe you had used peripheral addressing like saying L PIW instead of L IW. Then increasing process image will not decrease the scan cycle. Maybe you have replaced peripheral addressing with process image addressing and in this case yes the scan cycle should be decreased (Bu still not that much I think). Check this FAQ and also check page 276 (Calculating the cycle time) in S7-400H manual. Best regards. H-H

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This contribution was helpful to 1 thankful Users Maha

7/28/2009 7:27 AM	Rate (0)
Maha Posts: 118 Rating: (4)	Hi H-H, Sure thts the only difference we made. before changing the Image Area, we changed our I/O's to Memeory Bits..just to chk it... so after tht change, the scan time reduced to 7ms.....thn we though tht, it started checking the Field I/O's for every scan..... then we got advise from our experts to change the image area....and put back the I/O's (replaced by M Bits) it started working. onre more thing, we are not using any L IW instruction, our application is all digital i/o's.
Maha Posts: 118 Rating: (4)
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7/30/2009 10:36 PM	Rate (0)
H-H Platinum Member Joined: 7/2/2008 Last visit: 8/7/2025 Posts: 928 Rating: (309)	Hello Maha, If your process image was limited to 256 and you had digital I/Os up to the address 700 then for sure the CPU would go in OB121 for each bit address outside 256. With a small calculation, 700(Bytes) - 256(Bytes) = 444(Bytes) = 444*8 (bits) which means 3552 interrupt of OB121(Programmingerror)in each cycle. I think this is the reason behind this much extended cycle time (60ms). I'm just lifted with a small note in my mind which is that your colleagues should have noticed the internal fault LED for programming error before investigating the scan cycle time. Best regards. H-H

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This contribution was helpful to 1 thankful Users Maha

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