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s7-300 analog value representation

Created by: Techy at: 1/24/2007 8:46 AM (5 Replies)

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1/24/2007 8:46 AM	Rate (0)
Techy Posts: 86 Rating: (2)	Hello, I am trying to get into the basics of that magic no. 27648 & some how not able to decode it.[:mad:] Here's the problem - I have S7-300 with AI card SM331-7Kf02,12bit resolution. Now the manual says that 12 bit resolution is stored as left-aligned. with RSB's padded with zero. My understanding is that a 12 bit resolution means 0 to 4095 OR 0FFF Hex. So when I put these in 16 bit format and shift by 3 bits, the number comes to 32767 & not 27648. Any one can throw some light on this. Thanks Techy[:D]
Techy Posts: 86 Rating: (2)
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1/24/2007 9:45 AM	Rate (0)
Wizard Diamond Expert Joined: 4/24/2006 Last visit: 9/14/2023 Posts: 8592 Rating: (1132)	Hello Techy, Have a look at the following manual: http://support.automation.siemens.com/WW/view/en/8859629 Chapter 5 describes the representation of analog signal. The value between 26748 and 32767 means an overshoot/overflow. Hope this will help. Best regards, Wizard

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1/24/2007 9:58 AM	Rate (0)
Techy Posts: 86 Rating: (2)	Wiz, Thanks,I did refer that manual. My point is where does that value of 27648( i.e. 20ma or 100%) come from? What's the mathematical formula to arrive it? Techy
Techy Posts: 86 Rating: (2)
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1/24/2007 10:54 AM	Rate (0)
Wizard Diamond Expert Joined: 4/24/2006 Last visit: 9/14/2023 Posts: 8592 Rating: (1132)	Hi Techy, I'm sorry but I don't know Perhaps someone else can explain. Best regards, Wizard

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1/24/2007 6:45 PM	Rate (0)
SteveA Posts: 297 Rating: (41)	I don't think there's anything magic about the number. It just allows overrange values without wasting too much resolution. The hex value for 27648 is 6C00h, which is a nice "even" hex value. Some of the built-in functions expect this range, like the scale/unscale routines and the PID loop. SteveA
SteveA Posts: 297 Rating: (41)
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1/25/2007 9:46 AM	Rate (0)
trap_64h Posts: 3 Rating: (0)	Maybe the number come from DACprecision (10,11.12 sign etc bits). Maybe an expertcan answer this question [;)] trap#64h
trap_64h Posts: 3 Rating: (0)
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