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Compare DB in 1200

Created by: Control Boy at: 3/20/2013 6:53 PM (7 Replies)

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3/20/2013 6:53 PM	Rate (0)
Control Boy Posts: 60 Rating: (0)	Hi, i need a function that compars two identical DB's ad if a difference is detected trigger an output. I looked at this link: /tf/WW/en/Posts/54866 This is exactly what I need but it doesnt compile in TIA for 1200 only in S7 Can anyone help convert this into soemting that will compile in TIA for a 1200 PLC? LAR1P#0.0//LoadstartaddressintoAR1 OPNDB1//openfirstDBviaDBregister1 OPNDI2//opensecondDBviaDBregister2 L64 LOOP:T#_Loop_counter//Loopcounter(TEMPvariabletypeINT) LDBB[AR1,P#0.0]//use"P#x.y"offsetifDB1'sstartaddressishigherthanDB2's LDIB[AR1,P#0.0]//use"P#x.y"offsetifDB2'sstartaddressishigherthanDB1's <>I JCCONT//leaveloopifdifferenceencountered(ordootherhandlinghere) +AR1P#1.0//bumpAR1by1Byte L#_Loop_counter LOOPLOOP JUEND CONT:NOP0//dowhateverneedstobedonehereifdifferenceencountered SQ5.0 // set output 5 if difference???? END:NOP0
Control Boy Posts: 60 Rating: (0)
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3/20/2013 9:30 PM	Rate (0)
0_Einstein Diamond Expert Joined: 7/7/2010 Last visit: 6/8/2023 Posts: 13373 Rating: (2185)	If you want to do this for some random DB, you may want to look in the S7-1200 system manual for the POINTER, ANY, andVARIANT types, and see how to pass your DBs into your FB for comparison. I might not be smart enough to solve it gracefully and might go for the hammer solution by passing in a pointer toeachDB's DBX0.0, it's length in bytes, and then compare byte by byte using absolute referencing. Or, you could fieldread from each, compare the results, and return the success or failure. I am not sure of the best solution. You could come up with a different approach. Add a checksum to every DB you will compare. Each time you make a change to a DB tag, update the checksum. Then you just verfiy the checksum values are the same in both DBs. Very quick. It distributes the comparison computation over all the DB tag changes instead of comparing the entire DB every time you need to validate.
	science guy
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3/20/2013 11:43 PM	Rate (0)
Control Boy Posts: 60 Rating: (0)	Hi Huggy, how do I add a checksum?? I think this will suit my requirement because I can use the different checksum to trigger my function.
Control Boy Posts: 60 Rating: (0)
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3/21/2013 12:37 AM	Rate (0)
0_Einstein Diamond Expert Joined: 7/7/2010 Last visit: 6/8/2023 Posts: 13373 Rating: (2185)	B&C Ltd Hi Huggy, how do I add a checksum?? I think this will suit my requirement because I can use the different checksum to trigger my function. As you initialize your DBs during the first scan, add up the bytes and stuff that value into your checksum. It might bea DINT. Each time you update any portion of the DB, add the bytes for your DB that change to your checksum. You can do it any number of ways, but as long as you do it the same way to both DBs. Then you can say, ---\|DBprimary.Checksum <> DBsecondary.Checksum \|----\| FBxxx \|- There will always be a possibility that significantly different values can add up to the same checksum value, so if it becomes a problem, you can look at various weighting factors for different bytes of your DB so they always vary more dramatically. Like multiplying by some factor of 2 (like 4, 8, 16, etc) so the likelihood of various bytes somehow ending up with the same checksum result. Try it and let us know how it goes. This is also worth a look before you make your changes: http://en.wikipedia.org/wiki/Checksum
	Last edited by: huggy_d1 at: 3/21/2013 12:39 AM link to wiki for checksum science guy
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3/21/2013 7:38 PM	Rate (0)
Control Boy Posts: 60 Rating: (0)	Hi Huggy, whats the best way to add up the contents of the DB? I have some bool and words in the DB which are not directly compatible with the calculate block. I could convert all from word to int then use the calc but its a bit long winded as I'm using quite a few variables.... Is there a way I can read the whole block to compare like "db1.dbx 0.0 byte 30" SCL is probably more effeceint but I'm not up to speed with this yet Thanks.
Control Boy Posts: 60 Rating: (0)
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3/21/2013 8:55 PM	Rate (0)
0_Einstein Diamond Expert Joined: 7/7/2010 Last visit: 6/8/2023 Posts: 13373 Rating: (2185)	B&C Ltd Hi Huggy, whats the best way to add up the contents of the DB? I have some bool and words in the DB which are not directly compatible with the calculate block. I could convert all from word to int then use the calc but its a bit long winded as I'm using quite a few variables.... Is there a way I can read the whole block to compare like "db1.dbx 0.0 byte 30" SCL is probably more effeceint but I'm not up to speed with this yet Thanks. You can of course perform the initial calculation manually, assign the checksum that value, and update it anytime any value is altered within the DB. I would probably set them both to zero on first scan as they should be identical. If not, you can compare them tag by tag on first scan and trigger some correction required. Once you start running, anytime you assign a value to anything in your DBs, add that same value to your checksum. You can use the slice to add portions of larger tags like strings and characters. For example, myDB.tag1 is a DINT and I revise the value to 101010. In myDB.checksum, I add 101010 to checksum and save to checksum. If a different value is in my DB2.checksum, then you can force a correction. It is much easier to do with some more advanced trickery and using the pointer/any/variant datatype combined with the at overlay putting the values into an FBs localarray of the proper size. Then in SCL, it is a quick FOR loop to sum up the values. The system manual for the s7-1200 describes the above concepts. I have not done it so please feel free to share if you choose that path.
	science guy
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3/22/2013 8:51 AM	Rate (0)
Control Boy Posts: 60 Rating: (0)	Hi Huggy, I created a function that adds the parts of the DB I need together and multiply by 16 to create a checksum of a sort. On calling the function I set the checksum "check" value to the last DB value. If the contents of the DB changes I use a <> compare to set a flag. I works fine but is a bit inflexible and clunky. I'm sure there must be a way to check part of or the whole contents of a DB in a simple step like the S7-v5 options. I just haven't found it yet I will continue to try and get the STL working. Thanks for the help.
Control Boy Posts: 60 Rating: (0)	Last edited by: B&C Ltd at: 3/22/2013 8:52 AM
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3/22/2013 1:16 PM	Rate (0)
0_Einstein Diamond Expert Joined: 7/7/2010 Last visit: 6/8/2023 Posts: 13373 Rating: (2185)	B&C Ltd Hi Huggy, I created a function that adds the parts of the DB I need together and multiply by 16 to create a checksum of a sort. On calling the function I set the checksum "check" value to the last DB value. If the contents of the DB changes I use a <> compare to set a flag. I works fine but is a bit inflexible and clunky. I'm sure there must be a way to check part of or the whole contents of a DB in a simple step like the S7-v5 options. I just haven't found it yet I will continue to try and get the STL working. Thanks for the help. STL is not available in S7-1200. SCL is much better for this problem. As to the clunky-ness, look up the ANY and VARIANT types. If you pass in a pointer to the DB and the DB length, you should be able to set up an AT overlay into an array of bytes no larger than the size of your DB. Then loop through the array to create the checksum. A look at the documentation and some time getting the syntax right, and it should no longer feel so clunky. You may fault your plc watchdog timer several times during troubleshooting this method.......
	science guy
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