Skip to Content

Register Login

Industry Online Support

Technical Forum

Navigation

QW-PQW-SCAN TIME

Created by: vipz at: 12/22/2012 1:30 PM (6 Replies)

Rating (1)

Thanks 0

7 Entries

12/22/2012 1:30 PM	Rate (0)
vipz Gold Member Joined: 9/2/2012 Last visit: 8/15/2024 Posts: 711 Rating: (30)	Dear experts, am having some doubts related with analog INPUT and OUTPUT. Am using CPU 313C 2DP. 1. I read in documents that analog inputs and outputs are not writing to the process image area. is it right?? 2.In my project , scan time is 15ms. Am having 2 analog output in my project. say 256,258 both are written from same source whose value updates time is < 15ms. So how it will update the value to ANALOG OUTPUT ?? Ie will it depend the scan time or it will update continously?? 3. If am using PQW256 , QW258-- is there will be any difference in updating the values?? please suggest Best Regards Vipz
	With Best Regards VIPZ
Suggestion To thank Quote Answer

12/22/2012 1:40 PM	Rate (0)
dchartier Grandmaster-Expert Joined: 9/27/2006 Last visit: 9/13/2024 Posts: 12282 Rating: (2683)	Hello vipz; Using peripheral addresses (PIWxxx, PQWxxx) bypasses the process image table, as you stated. Since the analog values vary continuously by definition,the reading/writing is immediate, from your program instruction to the analog module, and you read/write only the present value of the analog signal. Since your instruction (for example, FC 105 "SCALE") reads a PIWxxx address, using this address adds nothing to the scan time. The only extratime you will require is access time to the analog module and the ADC/DAC conversion time, which is different in every analog module. Reading and writing to the process image (IWxxx, QWxxx) alsotakes up a bitscan time every CPUcycle, but stabilizes the digital I/Ovalues for one scan of the program. Hope this helps, Daniel Chartier
	Last edited by: dchartier at: 12/22/2012 1:51 PM
Suggestion To thank Quote Answer

12/22/2012 5:13 PM	Rate (0)
vipz Gold Member Joined: 9/2/2012 Last visit: 8/15/2024 Posts: 711 Rating: (30)	Thanks Dchartier .. regards Vipz
	With Best Regards VIPZ
Suggestion To thank Quote Answer

12/23/2012 1:08 AM	Rate (0)
Marko Bursic Platinum Member Joined: 9/8/2009 Last visit: 7/17/2023 Posts: 1410 Rating: (152)	dchartier Hello vipz; Using peripheral addresses (PIWxxx, PQWxxx) bypasses the process image table, as you stated. Since the analog values vary continuously by definition,the reading/writing is immediate, from your program instruction to the analog module, and you read/write only the present value of the analog signal. Since your instruction (for example, FC 105 "SCALE") reads a PIWxxx address, using this address adds nothing to the scan time. The only extratime you will require is access time to the analog module and the ADC/DAC conversion time, which is different in every analog module. Reading and writing to the process image (IWxxx, QWxxx) alsotakes up a bitscan time every CPUcycle, but stabilizes the digital I/Ovalues for one scan of the program. Hope this helps, Daniel Chartier Just small detail, reading and writing to peripheral (PIW, PQW,..) takes a little more time compared to process image (IW, QW,..) , but when dealing with analog I/O is always used peripheral R/W.

Suggestion To thank Quote Answer

12/23/2012 5:42 PM	Rate (0)
vipz Gold Member Joined: 9/2/2012 Last visit: 8/15/2024 Posts: 711 Rating: (30)	Ok MARKO. One more thing, Cant we use QW addresses instead of PQW ??
	Last edited by: vipz at: 12/23/2012 5:48 PM If am using like QW, will it take SCAN TIME+DAC- TIME also?? regards Vipz With Best Regards VIPZ
Suggestion To thank Quote Answer

12/24/2012 10:47 AM	Rate (1)
Marko Bursic Platinum Member Joined: 9/8/2009 Last visit: 7/17/2023 Posts: 1410 Rating: (152)	Very long time ago, when I begun to program I came to conclusion (it might not be the right) that for analog, counters,.. I have to R/W trough peripheral adress (PIW,PID,..) it won't work if you try with IW, ID, so I never tried again to use process image to access those signals. When you access trough peripheral address space, then the CPU asks a remote module what is the last good value, so it takes a quite more time compared to process image that resides in CPU memory. However ADC conversion time is much greater and new access to analog input module doesn't mean a new conversion command, conversions repeat continuously and you get only thelast measurment. PLC program would take very long timeif at anyIO access shouldwait for a conversion to complete making all useless, so it doesn't go that way. DAC is the same, altrough DAC performs very faster than ADC (conversion is quite immediately) still your plc program executes forward and it doesn't wait that output of DAC settles and then continues. Conclusion: when you need to read or write an analog value from/to analog card use peripheral addressing, but only once in your plc program, for all subsequent conversions, comparations and computations use memory area in plc, don't load the same ana. input twice or more via PIW. Also store the output value via PQW only once. You can use SCALE/UNSCALE functions,... You can also check execution times of different allocation load/transfer commans. Attachment s7300_parameter_manual_en-US_en-US.pdf (147 Downloads)

Suggestion To thank Quote Answer

12/24/2012 5:13 PM	Rate (0)
vipz Gold Member Joined: 9/2/2012 Last visit: 8/15/2024 Posts: 711 Rating: (30)	Thank you Marko for sharing the knowlege. . . regards Vipz
	With Best Regards VIPZ
Suggestion To thank Quote Answer

|

Share this page:

Share this page on...

Follow us on