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Bit access in array of Dint with variable index

Created by: Willem Remie at: 12/9/2014 5:00 PM (9 Replies)

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12/9/2014 5:00 PM	Rate (0)
Willem Remie Posts: 16 Rating: (2)	Hello all, I'm familiar with accessing a bit by means of SliceAccess. However, is there a possibility to access bits within a tag of data type Byte, Word, Dword or Dint with a variable index? So instead of: #Array[#ArrayIndex].%X2 Something of this form: #Array[#ArrayIndex].%X[#BitIndex] Thanks in advance
Willem Remie Posts: 16 Rating: (2)
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12/9/2014 5:18 PM	Rate (0)
William_B Posts: 5225 Rating: (1204)	--
William_B Posts: 5225 Rating: (1204)	Last edited by: William B. at: 12/9/2014 5:33 PM [delete message]
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12/9/2014 5:25 PM	Rate (0)
0_Einstein Diamond Expert Joined: 7/7/2010 Last visit: 10/31/2025 Posts: 16228 Rating: (2517)	Willem Remie Hello all, I'm familiar with accessing a bit by means of SliceAccess. However, is there a possibility to access bits within a tag of data type Byte, Word, Dword or Dint with a variable index? So instead of: #Array[#ArrayIndex].%X2 Something of this form: #Array[#ArrayIndex].%X[#BitIndex] Thanks in advance To access entire bytes or words (which you may not have asked, but might want to consider) .b0 (1st byte) .b1 (2nd byte) .w0 (1st word) .w1 (2nd word) Same for bits within any symbolic tag as your example myDWord.%X18 myDINT.%X24 myWord.%X4 The PLC platform does matter, but with the AT Overlay option, you can create an FB that picks out the bit you want from an input tag and returns the state of that bit.
	Last edited by: huggy_d1 at: 12/9/2014 5:27 PM science guy
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12/9/2014 9:01 PM	Rate (1)
William_B Posts: 5225 Rating: (1204)	Something of this form: #Array[#ArrayIndex].%X[#BitIndex] Hi there. What you ask can not be done directly (sorry). There is only indirect methods. I do not know if you would welcome us to help you brain-storm some methods. huggy_d1 has mentioned one such method which I will put 1st on the list.. 1. AT overlay method ..is available on S7-1500 / S7-1200 PLCs in some code blocks formats. In the S7-300 / S7-400 / ET200 it is available when using SCL coding. In short you could (for your case) create a DINT variable that has a "shaddow" variable that disassemble the bits into an array of bits. 2. Dedicated FC (or FB) ..that is capable of deconstructing the DINT into individual bits and feed back the bit result. Parameters associated with the code block could be.. * INPUT: The DINT to be evaluated (#Array[#ArrayIndex]) * INPUT: Bit number to extract from the DINT (#BitIndex) * OUTPUT: Bit result Several methods are possible to construct the above solution. One such.. 1. Load 1 into #CompareValue 2. Shift #CompareValue with #BitIndex positions. Save in #CompareValue 3. #Result := ( ( #Array[#ArrayIndex] WORD_AND #CompareValue ) <> 0 ) Helpful? w
William_B Posts: 5225 Rating: (1204)
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12/9/2014 9:32 PM	Rate (2)
William_B Posts: 5225 Rating: (1204)	..design idea attached when using AT overlay - when available. Note that the DINT is now DWORD. For handling DINT a convert network is required. w Attachment Design idea_extract bit from array of DWORDs.jpg (1674 Downloads)
William_B Posts: 5225 Rating: (1204)
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12/16/2014 4:45 PM	Rate (0)
Willem Remie Posts: 16 Rating: (2)	Thanks William for the extensive examples provided. I considered the AT overlay as well and i tried it before. I will probably use this option as a solution. It's nice to know the different options available. It makes no sense to me why this still not can be done directly though. Or is this still a function to be implemented in TIA portal? It would be a really time saving feature, at least for me. Especially because other programming environments support this symbolic variable index access. Thanks for your help!
Willem Remie Posts: 16 Rating: (2)
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This contribution was helpful to 1 thankful Users William_B

12/16/2014 5:04 PM	Rate (0)
William_B Posts: 5225 Rating: (1204)	Willem Remie Thanks William for the extensive examples provided. I considered the AT overlay as well and i tried it before. I will probably use this option as a solution. It's nice to know the different options available. It makes no sense to me why this still not can be done directly though. Or is this still a function to be implemented in TIA portal? It would be a really time saving feature, at least for me. Especially because other programming environments support this symbolic variable index access. Thanks for your help! I hear you and having worked on some other environments, I can agree with you. But hey... we work with what we have; if you can adapt - then you'll be doing fine. The main thing is to get the job done. Note that in the Siemens family there are various options - depending on what processor you use under which software. Greetings. w Late Note: You may wish to page to the following tip: [LINK] There are detail notes in following posts that might be of value.
William_B Posts: 5225 Rating: (1204)	Last edited by: William B. at: 12/16/2014 5:11 PM add late note
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12/16/2014 5:51 PM	Rate (0)
Willem Remie Posts: 16 Rating: (2)	Cant agree more but i'm always searching for the most effiecent and lean solution. I already found the other post, but thanks for the link anyway. With the AT overlay, is itnecessary to disable the 'optimized block access' option? In case we dont want to know the status of a bit inside a element but instead set a bit. This trick will work exactly the other way arround right? Just out of curiosity, in case of SCL and STL, does there exist a 'cleaner' solution in case we use indirect adressing?
Willem Remie Posts: 16 Rating: (2)	Last edited by: Willem Remie at: 12/16/2014 6:13 PM
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12/16/2014 9:21 PM	Rate (1)
William_B Posts: 5225 Rating: (1204)	Willem Remie Cant agree more but i'm always searching for the most effiecent and lean solution. I already found the other post, but thanks for the link anyway. With the AT overlay, is itnecessary to disable the 'optimized block access' option? [1] In case we dont want to know the status of a bit inside a element but instead set a bit. This trick will work exactly the other way arround right? [2] Just out of curiosity, in case of SCL and STL, does there exist a 'cleaner' solution in case we use indirect adressing? [3] Willem, [1] Under TIA V13 there is a little trick that allows you to use AT OVERLAY even if the block is set to "optimised" structure (S7-1200 and S7-1500). In the RETAIN column you select "Set in IDB". [2] Within the code block working with the primary variable or the "shadow" variable is the same. You could have the DWORD as primary variable or have the ARRAY as primary variable. You can even have a STRUCT of individual bits (primary or shadow). But remember that outside the code block, only the primary variable can be accessed. So ensure when selecting what the primary variable is, that this is the one that the other coding should have access to. [3] STL and SCL over a few other options. Remember though that the S7-1200 does not offer STL. IMHO - I would not say it the alternate methods are cleaner when we speak of the S7-1x00 CPUs. The S7-300 and S7-400 has other methods for indirect addressing and absolute addressing, but because this lead to complications we are trying to move away from this. For now, what we have spoken about is maybe the safest and most efficient. Do remember though that there are still masking (Word-sized AND / OR ) and shift/rotate commands. Lets say you want to read Bit5 (user defined position), you could ROTATE the DWORD value x5 spaces to the right and read the Bit31. Best of luck to you. w
William_B Posts: 5225 Rating: (1204)
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12/28/2017 12:51 PM	Rate (1)
Sava Experienced Member Joined: 9/6/2011 Last visit: 9/23/2025 Posts: 22 Rating: (9)	Maybe this would be useful for someone. The main idea is to use AT overlay access. Attachment SetBitInWord.png (931 Downloads)

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