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Data Block problem.

Created by: gumis at: 4/25/2009 12:07 PM (5 Replies)

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4/25/2009 12:07 PM	Rate (0)
gumis Posts: 45 Rating: (0)	Dear all, I haveproblem with Move instructions and Data Blocks. Firstly, I open Data Block. Then I use Move instruction. IN operand is a constant, for instance, W#16#3010 and OUT is DBW0 - first word in opened data block. The problem is, low significant byte becomes high significant byte in data block where I write. The second problem is, when I write W#16#3010 value to DBW1 - second word in opened data block, I receive W#16#1000. May I write constant value using Move instr.? Please,help me.
gumis Posts: 45 Rating: (0)	Last edited by: gumis at: 4/25/2009 12:31 PM
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4/26/2009 1:02 AM	Rate (0)
Sydney Gold Member Joined: 1/4/2008 Last visit: 12/22/2025 Posts: 703 Rating: (98)	Hello, SIMATIC S7is byte-oriented so the read and write is little bit different then in some others control systems. If you just look into DB it couldlooked switched but if you load the first word from the DB (DBW0)you get the same value which you transfer there. The second case is also about the byte orientation - the DBW1 in S7 isn't second word like in S5, the number in address is always byte number and it doesn't matter if you use word or doubleword access. Regards Sydney

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4/26/2009 5:58 AM	Rate (0)
dchartier Grandmaster-Expert Joined: 9/27/2006 Last visit: 1/14/2026 Posts: 12481 Rating: (2722)	Hello gumis; Sydney's description is right-on. One suggestion if you wish to avoid the adrressing issue would be to use symbolic addressing. Simply use the Symbol table to attribute a symbol to your DB, then you can use the individual names of the DB register. Instead of DB1.DBW0 you could access 'DB_Motor1'.CntrlWord, for example. Step 7 would take care of locating the correct address. Hope this helps, Daniel Chartier

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4/26/2009 11:44 PM	Rate (0)
gumis Posts: 45 Rating: (0)	Let's assume I write first word value to DBW0. Does it mean, more significantbyte from first word becomes BYTE0 in data block and lower byte becomes from first word BYTE1 in data block? Let's assume I write second word value to DBW1 does it mean I will write to BYTE1 and BYTE2 of this data block? Could you recommend me some Siemens documents about that?
gumis Posts: 45 Rating: (0)
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4/27/2009 2:18 PM	Rate (0)
Sydney Gold Member Joined: 1/4/2008 Last visit: 12/22/2025 Posts: 703 Rating: (98)	Yes, generally in S7 if youhave word data type, the higher byte from the numberis stored in the lower byte in memory. It can look strange but it's just first sight. Consider this: When you declarea variable with the lengthmore than byte, it always starts on the even byte number in memory (so normally you shouldn't use DBW1 for example). After declaration the system automatically assigns the address. When you use symbolic addressing in your program you don't have to take care about theaddress and storage addressing system in memory (DB). When you declare a variable of the type WORD and transfer there some valuethen you see thevalue exactly the same in monitoring of the DB. Only you have to take care is when you have a communication with some word-oriented system (for example S5 or frequency changer) and send/receive some status words which you acces by bits, in this case the byte order is switched, but it's just about how you declare the bits in the communication DB. Concerning the literature: it's good to start with the documentation which comes with the Step7, after installationit can befound in Startup menu under SIMATIC->Documentation. For basicsI suggest these: STEP 7 - Working with STEP 7 STEP 7 - Programming with STEP 7 STEP 7 - Statement List for S7-300 and S7-400 eventually STEP 7 - Function Block Diagram for S7-300 and S7-400 STEP 7 - Ladder Logic for S7-300 and S7-400 and in addition there are other options for Step7 programming like SCL, S7 Graph, CFC and so on... Regards Sydney
	Last edited by: Sydney at: 4/27/2009 2:20 PM
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4/27/2009 8:13 PM	Rate (0)
gumis Posts: 45 Rating: (0)	Thank you. I have used symbolic addressing and it works fine. Blue Moderator New question was inserted as a new thread: Data Block problem: more or less significant byte?.
gumis Posts: 45 Rating: (0)	Last edited by: Blue Moderator at: 4/27/2009 9:56 PM Created as a new thread. Last edited by: gumis at: 4/27/2009 8:56 PM Last edited by: gumis at: 4/27/2009 8:51 PM
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