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Retentive M memory that is saved ONLY in RAM

Created by: Stockhlam at: 5/27/2011 5:04 PM (6 Replies)

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5/27/2011 5:04 PM	Rate (0)
Stockhlam Posts: 8 Rating: (0)	Hello! In the help about data retentivity I found this: "[i]Changing the range MB0 to MB13 to be retentive enables a special feature that automatically saves these locations to the EEPROM on power down.[/i]" and "[i]The first 14 bytes of M memory (MB0 to MB13), if configured to be retentive, are permanently saved to the EEPROM when the CPU module loses power.[/i]" Does that mean that I have to define exactly these 14 bytes as retentive to save them to the EEPROM or they will be saved even if the defined range overlaps this 0..13 range? Where the data will be saved if I define one of these ranges as retentive: MB0..MB20, MB0..MB5, MB7..MB13 ? I want to save about 16 Marker memory bytes on power loss but only in the RAM and NOT in the EEPROM, is that possible? I'm using CPU222 v2.01, and for some reason I can't define a retentive range that starts at MB9 or more - compiler gives out a warning 2756 "Retentive Range is illegal for the PLC type selected".
Stockhlam Posts: 8 Rating: (0)
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5/27/2011 5:29 PM	Rate (0)
umencho Grandmaster Joined: 3/21/2006 Last visit: 10/16/2025 Posts: 10239 Rating: (1183)	Hello, Read there about retentive memory of your type of CPU and what is possible to do. http://support.automation.siemens.com/WW/view/en/431638 best regards, Hristo Mihalev

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This contribution was helpful to 2 thankful Users Reva P Stockhlam

5/29/2011 3:44 PM	Rate (0)
Stockhlam Posts: 8 Rating: (0)	Following one of your links I found a sentence that says that ANY byte that's set as retentive in this 0..13 range will be written to EEPROM. That clears things up a bit. Thanks! Now I could move the used M bytes to higher addresses but there is the problem with illegal range. I know that this CPU supports M0.0 to M31.7, but I don't see anything in the manual about invalid retentivity ranges. For example setting range like MB14..MB15 doesn't work - compiler shows that previously mentioned warning. But using range MB6..MB15 (or any other that starts below byte 9) will not give a warning. Am I missing something here?
Stockhlam Posts: 8 Rating: (0)
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5/31/2011 11:50 AM	Rate (1)
a1b2c3 Posts: 154 Rating: (17)	Hello, If the first 14bytes of M memory as retentive memory are not enough, I recommend you to use the library from the following faq: http://support.automation.siemens.com/WW/view/en/17471561 Best regards
a1b2c3 Posts: 154 Rating: (17)
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5/31/2011 12:50 PM	Rate (0)
Stockhlam Posts: 8 Rating: (0)	There is no shortage of retentive memory. The problem is that I can't choose the range I want. If I understand correctly, some of the M memory is saved in EEPROM and some of it is kept in RAM that is povered by a supercapacitator. I would like NOT to use the EEPROM and save the variables only in RAM. Since the first bytes get automatically saved in EEPROM I could start my addressing from M14.0 instead of M0.0 (and use range MB14 to MB29), but the compiler says that this range can't be retentive on my CPU. If I define the MB0-MB30 as retentive, there will be no warning. Only if the range starts at MB9 or higher byte. Ok, when I'll have access to the CPU I will try the following: - use the range I want ignoring the warning and test if the values are saved. - or set some of the unused memory as retentive, hoping that before writing CPU actually checks if the memory bits have changed.
Stockhlam Posts: 8 Rating: (0)
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5/31/2011 1:12 PM	Rate (0)
a1b2c3 Posts: 154 Rating: (17)	OK, if you will store the MB14 up to MB29 in RAM. You have to write in the system data block: Data Area: MB; Offset: 14 and Number of Elements: 16 (not 29) If you write 29, you have a area from MB14 to MB42 and this is not possible, therefore you get the warning. You have to insert the numbers of elements (how many bytes) you will store, not the last MB. I hope this help you.
a1b2c3 Posts: 154 Rating: (17)
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This contribution was helpful to 1 thankful Users Stockhlam

5/31/2011 2:09 PM	Rate (0)
Stockhlam Posts: 8 Rating: (0)	Oh, I admit I didn't notice that the second number is element count and not the last element (feel soooo stupid right now). But I remember also trying various combinations of small values, like Offset=12/Elements=14 (thinking of MB12-MB14), Offset=10/Elements=14,etc and still getting the warning. And what's strange - now all these values are accepted, everything works exactly as you said. Maybe I was doing something else wrong :/ Hopefully in a few days I'll get access to the CPU and change the range. Until then thanks to UMENCHO for sending me in the right direction and you, a1b2c3, for opening my eyes!
Stockhlam Posts: 8 Rating: (0)
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