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M x.x vs DB.DBX x

Created by: amrmabrouk at: 7/24/2010 8:29 PM (4 Replies)

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7/24/2010 8:29 PM	Rate (0)
amrmabrouk Posts: 13 Rating: (0)	Dear all I am working on a project and I faced a problem while trying to add some functionality. In brief,I want to use DB bits instead of memory bits. Are there any technological differeneces(accumulator usage, execution time , etc) between the two???????? thanks in advance
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7/25/2010 1:29 AM	Rate (1)
dchartier Grandmaster-Expert Joined: 9/27/2006 Last visit: 6/9/2023 Posts: 12037 Rating: (2597)	Hello amr; There is a price to pay when using DBs. It is clearly more cosatly to do so if the CPU selected is of the lower range: almost double the access time in a CPU 315C, whereas in a CPU 319 the access time is the same. See the attached table, from the S7-31xC Instruction list manual: Hope this helps, Daniel Chartier Attachment ExecTimes.jpg (94 Downloads)

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7/25/2010 8:49 AM

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Ayman Elotaify

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Joined: 5/28/2008

Last visit: 6/6/2023

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Hello Amr,
I would like to add small info to what Daniel Chartier said.
There's limition on using flags for CPUs, So you have to be careful when using flags. I believe that the available number of DBs will be suitable for most of the applications.
For example:

CA01

Standard CPUs
CPU Flags; No., Max. / Address DB; No., Max
CPU312 256 byte/ MB0 – MB255 1024
CPU314 256 byte/ MB0 – MB255
1024
CPU315-2 DP 2048 byte/ MB0 – MB2047
1024
CPU315-2 PN/DP 2048 byte/ MB0 – MB2047
1024
CPU317-2 DP 4096 byte/ MB0 – MB4095 2047
CPU317-2 PN/DP 4096 byte/ MB0 – MB4095 2048
CPU319-3 PN/DP 8192 byte/ MB0 – MB8191 4095

Compact CPUs
CPU Flags; No., Max. / Address DB; No., Max
CPU312C 128 byte/ MB0 – MB127 511
CPU313C 256 byte/ MB0 – MB255 511
CPU313C-2 PtP 256 byte/ MB0 – MB255 511
CPU313C -2 DP 256 byte/ MB0 – MB255 511
CPU314C-2 PtP 256 byte/ MB0 – MB255 511
CPU314C -2 DP 256 byte/ MB0 – MB255 511

Hope this helps.

Ayman Elotaify

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7/25/2010 10:00 AM	Rate (0)
amrmabrouk Posts: 13 Rating: (0)	thanks all for your help but I have another question what is the max no of bytes to be used by all DBs in CPU 315-2DP?????? thanks in advance
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7/25/2010 10:23 AM	Rate (0)
IBN-Service Posts: 8946 Rating: (995)	16kByte for each DB. All together, not larger than the CPU-Memory of course! There is also a possibility of unlinked DB, but that is not usefull in your application here.
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