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SFC14 - reading 4 bytes

7/9/2010 11:46 PM	Rate (0)
schnek Bronze Member Joined: 11/13/2007 Last visit: 3/20/2025 Posts: 41 Rating: (0)	Hello all, In my application, I have found out, that when I read 4 consistent bytes with sfc14 (cpu S7 315-2DP) from a dp slave the second word returns some random data. When I access this second word directly over PIW everything seems to work fine. After some searching on this support page I found that some CPUs do not support reading/writing 4 bytes with SFC14/15, but the 315-2DP should work fine as to this: http://support.automation.siemens.com/WW/view/en/21628388 ...!? I would like to ask, if someone could explain me, where could be the problem and what is the reason why some CPUs do not support reading/writing 4 consistent bytes with SFC14/15... Thank you very much for your hints.

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7/10/2010 2:34 AM	Rate (1)
dchartier Grandmaster-Expert Joined: 9/27/2006 Last visit: 3/22/2025 Posts: 12344 Rating: (2707)	Hello schnek; You are right that you cannot use SFC14/15 to access 4 bytes of consistent data (3 bytes or 5 bytes and higher are allowed), but this appliesin any Siemens S7 master. This is seen in the following FAQ: http://support.automation.siemens.com/WW/view/en/245111 Slaves with 4 bytes address area can NOT* be processed with the SFC14/15. Such slaves include ET200U,M,L,B,C and X. Here via periphery direct access you can read (PID) and write (PQD) data.* Hope this helps, Daniel Chartier
	Last edited by: dchartier at: 7/10/2010 2:34 AM
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7/10/2010 5:38 PM	Rate (0)
schnek Bronze Member Joined: 11/13/2007 Last visit: 3/20/2025 Posts: 41 Rating: (0)	Thank you very much for your answer, dchartier. I am accessing in such way not an ET200 slave but one axes in CU320 control unit... in fact it is a supply unit that has only 2 PZDs to read. Does this hold good for all slaves, that when you only have two words to read, you have to use standard Load instruction?
	Last edited by: schnek at: 7/10/2010 5:42 PM Last edited by: schnek at: 7/10/2010 5:41 PM
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