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image table of analog I/Os

Created by: deepak.samal05 at: 9/17/2010 3:56 PM (3 Replies)

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9/17/2010 3:56 PM	Rate (0)
deepak.samal05 Posts: 65 Rating: (0)	hello sir, [schnippschnapp] Another question is related with the image table of analog I/Os. Since s7-300 series controllers does not have Image table for analog I/Os so it is using analog module for memory and bcos of that i am using PIW/PWQ.(PIW256).Is the explaination correct??????? [schnippschnapp] Splitted from Byte allocation for digital and analog I/Os.
deepak.samal05 Posts: 65 Rating: (0)	Last edited by: O_Moderator at: 9/20/2010 2:41 PM new subject after splitting
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9/17/2010 4:32 PM	Rate (0)
dchartier Grandmaster-Expert Joined: 9/27/2006 Last visit: 12/20/2024 Posts: 12307 Rating: (2695)	Hello deepak; [schnippschnapp] Another question is related with the image table of analog I/Os. Since s7-300 series controllers does not have Image table for analog I/Os so it is using analog module for memory and bcos of that i am using PIW/PWQ.(PIW256).Is the explaination correct??????? PIW and PQW addresses are peripheral, which means direct read/writing of the analog channels, when the program needs it. The image tables (for digital inputs and outputs) are updated cyclically be the OS of the CPU, the inputs at the beginning of the CPU scan, the outputs at the end of the scan. These values are maintained throughout the scan cycle, until the next update, so the values are considered stable during the scan. The program can then use the input image table values for evaluation of the logic, and write the resulting digital outputs all at once. Analog values by definition do not remain stable, and may vary during the scan. When you access a PIW address you are doing an Immediate Read, capturing the analog value that exists in the analog channel at the exact moment the program will use it. When you send a value to a PQW address you write the outputvalue at the exact moment the program sends it to the output channel. [schnippschnapp]Hope this helps, Daniel Chartier
	Last edited by: O_Moderator at: 9/20/2010 2:44 PM Last edited by: dchartier at: 9/17/2010 4:35 PM
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9/24/2010 11:09 AM	Rate (0)
Oscar_83PL Bronze Member Joined: 10/22/2008 Last visit: 12/14/2024 Posts: 370 Rating: (5)	So it is possible to access analog values on the begin and end cycle by adressing them with IW and QW? PIW&PQW imidiet and IW and QW for begin and end cycle?

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9/24/2010 3:05 PM	Rate (0)
dchartier Grandmaster-Expert Joined: 9/27/2006 Last visit: 12/20/2024 Posts: 12307 Rating: (2695)	Hello Oscar; If you do not mind waiting for the OB1 cycle to complete, yes you can access analog values through IW and QW addresses, as part of the process images tables. Just make sure the addresses used are inside the limits of the process image tables for your CPU (see the technical specs). In some CPUs (S7-400) you can even use image partitions (PII), which divide the image tables into sub-tables that can be updated at different cycles. This way your scan cycle is made asynchronous, and predictable. This is used for PCS 7 projects, as I have seen. For more information, have a look at the following FAQ: Where and when do you need peripheral addressing? http://support.automation.siemens.com/WW/view/en/18325417 Hope this helps, Daniel Chartier

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