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Data Block Access in Step7

Created by: Siemens_MI at: 6/7/2010 9:01 PM (5 Replies)

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6/7/2010 9:01 PM	Rate (0)
Siemens_MI Member Joined: 6/7/2010 Last visit: 3/27/2023 Posts: 11 Rating: (0)	I am working on a project in which many of the FC/FBs need continual access to external data blocks,rather than input/output parameters(due to the architecture of the program.) I'm looking for a description of how a Step7 FB/FC manages the address registers while accesses data blocks. I can't seem to find this in the general user documentation. Any help would be appreciated. Thanks

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6/8/2010 2:36 AM	Rate (1)
dchartier Grandmaster-Expert Joined: 9/27/2006 Last visit: 5/28/2023 Posts: 12024 Rating: (2594)	Hello Siemens_MI; Let me see if I understand your question correctly. Say you create a Data Block for motor control data, and give it number 12. Inside, you create registers where your data will be written and accessed. Say, Word register address 2,0 will be named Shaft_Speed and will be used as an INT; say DWord register 16.0 will be named Winding_Temp and will be used as a REAl (floating-point) value. Anywhere in your program, in any FC or FB that nees the value, you can program (in STL, for example; the cooresponding LAD instruction is MOV) L DB12.DBW2 // load the value at DB12.DBW2 into ACC1 T MW 256 //transfer value in ACC1 to MW256 and the value of the register Shaft_Speed will be transfered to MW256 for any calculation you need. This is an example of what is called "absolute addressing", Now, if you had planned to givew DB12 a symbolic name (in the Symbols table), possibly PUMP1, then the same operation could have been written (symbolically)as L PUMP1."Shaft_Speed" //load value of DB12.DBW2 into ACC1 T MW 256 //transfer value in ACC1 to MW256 You can now also access the symbolic address PUMP1."Winding_Temp" symbolically, since DB12 has been attributed its symbolic name. L PUMP1."Winding_temp" ...will mean exactly the same as L DB12.DBD16... Does this answer your question? In any case, when you installed Step 7, documents were loaded into your computer (in Simatic -->Documentation). Look for the manuals "Working with Step 7-First steps" and "Programming with Step 7", they will help you greatly to understanmd memory areas available in the S7 CPUs, and how to access them. Hope this helps, Daniel Chartier
	Last edited by: dchartier at: 6/8/2010 2:38 AM
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6/8/2010 6:28 AM	Rate (0)
fritz Diamond Expert Joined: 10/7/2005 Last visit: 5/29/2023 Posts: 2965 Rating: (1025)	Hello Siemens_MI I could not have explained fully qualified DB access better than Daniel did, but wonderif youwere after the effects that your planned (fully qualified or partial) DB accesses have on the contents of the Address Registers (AR1 & AR2). If that's the case, then the short answer is: None Having said that, AR1 and/or AR2 (as well as DB 1 & 2 Registers) can be "overwritten" with certain (sometimes seemingly unrelated) operations and you can find more info about in the FAQ below (note too the "Avoiding errors when calling blocks" reference to the Step 7 help in it): Which operations overwrite the content of ACCU or registers in STEP 7? I hope thishelps P.S: Address and DB registers as well as accumulator content is saved (and restored afterwards) in case a higher priority Interrupt 'kicks in" while your Block is executed(see HERE for more).
	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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6/10/2010 4:20 PM	Rate (0)
Siemens_MI Member Joined: 6/7/2010 Last visit: 3/27/2023 Posts: 11 Rating: (0)	Thanks for the info, and the pointer to the documentation. That helped some; however, I still find myself confused regarding what the performance penalty is for using global data blocks with arrays in them. When you have an SCLstatement that contains something like: "DB_GLOBAL_INPUT".MACHINE[MACHINE_NO].INPUT_01 what happens when you reference this? Then when the next statement contains something like: "DB_GLOBAL_OUTPUT".MACHINE[MACHINE_NO].OUTPUT_01 what registers have to be re-loaded? Is this process less expensive than passing these values by parameter?

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6/10/2010 9:31 PM	Rate (0)
fritz Diamond Expert Joined: 10/7/2005 Last visit: 5/29/2023 Posts: 2965 Rating: (1025)	Hello Siemens_MI Siemens_MI When you have an SCLstatement that contains something like: "DB_GLOBAL_INPUT".MACHINE[MACHINE_NO].INPUT_01 what happens when you reference this? Then when the next statement contains something like: "DB_GLOBAL_OUTPUT".MACHINE[MACHINE_NO].OUTPUT_01 what registers have to be re-loaded? Good question, I'm not too sure if this really should really worry you though. If you are programming in SCLthe "gory details"of AR and DB register usage are hidden from you. Since SCL compiles into STL, one way I can think of to to find out is to write your SCL logic, compile it, move the the SCL source file somewhere else and then open the block which will now show you the created STL code. Siemens_MI Is this process less expensive than passing these values by parameter? Couldn't tell you (never worried about this as I rarely touch SCL) and it would also depend if you pass on an elementary or complex data in combination with the type ofBlock you use(FC or FB). I can tell you though that SCLwill create "surplus" STL logic when compiled, so if you are really concerned about runtime performance or memory usage you can't beat programming in the language that rules over all the other ones (i.e. STL). I hope this helps somewhat and perhaps others have more ideas on this matter.
	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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6/11/2010 1:53 AM	Rate (0)
Siemens_MI Member Joined: 6/7/2010 Last visit: 3/27/2023 Posts: 11 Rating: (0)	Thanks for the reply. I tried the trick you mentioned about removing the source file and opening the block. That provided some interesting details. When I have more time, I'll set up an example to see how the compiler optimizes consecutive accesses to different global DBs. I'll post anything I find on this thread.

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This contribution was helpful to 2 thankful Users fritz dchartier

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