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Memory Representation

Created by: Saradhi at: 9/6/2012 8:12 AM (5 Replies)

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9/6/2012 8:12 AM	Rate (0)
Saradhi Posts: 303 Rating: (3)	Dear All, I am little bit confused memory mapping in Siemens PLC's. If i pass a value 1 to MB0, it will occupy M0.0. ---> No issue If i pass a value 1 to MW0, it will occupy M1.0. If i pass a value 1 to MD0, it will occupy M3.0. What is reason to map the memory like this.
Saradhi Posts: 303 Rating: (3)
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9/6/2012 8:26 AM	Rate (0)
Aret Posts: 2348 Rating: (263)	It looks like M0.7......0 M1.7......0 M2.7......0 M3.7......0 M4.7......0 .... So when you write 1 to MW0 you write it to M0.7.....0.0 M1.7.....1.0. In this case 1 is written to lowest bit - i.e. M1.0
Aret Posts: 2348 Rating: (263)
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9/6/2012 8:39 AM	Rate (0)
Saradhi Posts: 303 Rating: (3)	Yes, that is i am asking. instead of representing like this M0.7.....0.0 M1.7.....1.0 The following order is benefitable for programmers when they requie to communicate with other controllers. M1.7.....1.0M0.7.....0.0
Saradhi Posts: 303 Rating: (3)
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9/6/2012 9:55 AM	Rate (0)
fritz Diamond Expert Joined: 10/7/2005 Last visit: 11/30/2023 Posts: 2997 Rating: (1040)	"The following order is benefitable for programmers when they requie to communicate with other controllers. M1.7.....1.0 M0.7.....0.0" only if the "other controller" operates with the Little Endian format . Siemens PLC's operate withthe Big Endian format, but do offer you STL commands like CAW (swap Bytes in a Word) and CAD (swap Bytes in a DoubleWord) to accomodate handling of Little Endian formatted values. I hope this helps and you'll find more info about the Little vs Big endian differences at http://en.wikipedia.org/wiki/Endianness
	Last edited by: fritz at: 9/6/2012 10:04 AM removed non working quotes Cheers fritz The advice and opinion given in this thread is that of the author and does not necessarily reflect the views of I S Systems Pty Limited. To find out how I S Systems can help you with your automation needs please visit www.issystems.com.au.
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This contribution was helpful to 1 thankful Users dchartier

9/6/2012 2:48 PM	Rate (0)
GuilhermeM Advanced Member Joined: 1/10/2012 Last visit: 11/9/2022 Posts: 24 Rating: (1)	Hi there, Some info: https://support.automation.siemens.com/WW/llisapi.dll?func=cslib.csinfo&lang=en&objid=22127697&caller=view /tf/WW/en/Posts/66038
	Best Regards, Guilherme Marques
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9/14/2012 8:47 AM	Rate (0)
Saradhi Posts: 303 Rating: (3)	Thank You
Saradhi Posts: 303 Rating: (3)
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