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Created by: RickM at: 10/13/2010 11:16 PM (8 Replies)

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10/13/2010 11:16 PM	Rate (0)
RickM Posts: 17 Rating: (0)	I am doing my first major program using Step 7 is not a big program. The program consist on checking the feedback on values, opening values, turning pumps on, off and some analog values. On the programs that I have as examples I see they use a lot of instance data blocks and temporary variables. I would like to stay always from instance blocks and maybe temporary variables. I know I will end up using temporary variables. I would like to use only inputs and outputs with M memory variables. Do you guys think that a good idea or do I need to use instance data block. If so can someone give me a better description of instance data block then the book does? I understand that the information that is in that FB is store in a DB. Thanks Rick
RickM Posts: 17 Rating: (0)
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10/14/2010 12:49 PM	Rate (0)
Denilson Pegaia Platinum Member Joined: 9/23/2005 Last visit: 9/13/2024 Posts: 4347 Rating: (1446)	Hi, there is a short description: temp var: don't retain the value after run the block (run cycle). Use only to move data from ainstruction to other inside a block. stat var: retain values between cycles. Use this variables to retain values that are independts in each block call. memory var: retain values between cycles. Use this variables to store variables common to all block calls.
	Denilson Pegaia Siemens Industry Sector Technical Support
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10/14/2010 2:16 PM	Rate (0)
RickM Posts: 17 Rating: (0)	In your experience do you believe i can get away without using those variables and the instance data block?
RickM Posts: 17 Rating: (0)
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10/14/2010 4:23 PM	Rate (0)
dchartier Grandmaster-Expert Joined: 9/27/2006 Last visit: 9/16/2024 Posts: 12282 Rating: (2684)	Hello Rick; Step 7 is flexible enough that you can get away with most anything, as long as you do not directly contradict fundamental rules, One of them is that any FB you generate must be accompanied by an Instance DB every time it is called in your program. So, in your program,. only use FCs, which do not have associated IDBs, and program all your values in globalM registers, or DBs, and you will be OK. We'll just put it down to your "programming style"... Hope this helps, Daniel Chartier

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10/14/2010 4:56 PM	Rate (0)
RickM Posts: 17 Rating: (0)	Thanks I will use FCs instead of FBs for now until i get the hang of the FBs and there IDBs.
RickM Posts: 17 Rating: (0)
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10/15/2010 3:51 PM	Rate (0)
RickM Posts: 17 Rating: (0)	What about being able to use SFB and SFC? I would like to used the TON and TOF function block can i used them with out IDBs?
RickM Posts: 17 Rating: (0)
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10/15/2010 4:01 PM	Rate (0)
dchartier Grandmaster-Expert Joined: 9/27/2006 Last visit: 9/16/2024 Posts: 12282 Rating: (2684)	Hello Rick; Ah no, so sorry... as soon as you want to create or use an FB (be it a standard function block, SFB) you are required to use the IDB. You could create your own FC instead of using the FB. But let's say you want to create a timer , you will need to memorize the preset and accumulated value of this function somewhere, so the timer can act while other blocks are being processedand you will need to connect the DBs or M-registers to this FC... Please explain what disturbs you in the IDB usage? It is simply a memory block that is assigned to a FB, so that a specific structure is used every time you call the FB. The best way for you to understand this is to call one SFB in your program, and assign it one IDB (when you call the SFB in Ladder, on top of the box of the SFB there appears a red ???; if you click on it it will ask you to name the DB, just give it an unused number in your program). The repeat the same operation in the next network, with a different DB. Open the DBs and look at the structure defined within. You will see it is repeated, so you can assign different values to this instance of the SFB, while keeping the structure identical. That way you may use the SFB "TON" for example with different time presets in different parts of the program. Hope this helps, Daniel Chartier
	Last edited by: dchartier at: 10/15/2010 4:04 PM
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10/15/2010 4:32 PM	Rate (0)
RickM Posts: 17 Rating: (0)	That make better sense to me . Thanks for your help. It i not use to doing programming in Step 7 i can follow the logic but not to comforter with the IBDs that all.
RickM Posts: 17 Rating: (0)
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10/15/2010 5:21 PM	Rate (0)
dchartier Grandmaster-Expert Joined: 9/27/2006 Last visit: 9/16/2024 Posts: 12282 Rating: (2684)	Hello Rick; In my experience, comfort will come with familiarity, no problem. IDBs are not exclusive to Siemens S7, even if in our case it is obvious when you use one. If you program a Timer or Counter variable in Rockwell PLCs, you always know which bit of the Timer/Counter structure holds the .DN information, which address offset will get you the .ACC value. This is because everytime you use such a variable a small standard structure is associated with it, for that instance... same, same, but different! Just experiment a bit when you find yourself facing such a problem and ask here for more explanations, if need be. Hope this helps, Daniel Chartier
	Last edited by: dchartier at: 10/15/2010 5:21 PM
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