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Latching by temporary varibale inside FB or FC- recomended?

Created by: KayBee at: 4/16/2011 2:20 PM (12 Replies)

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4/16/2011 2:20 PM	Rate (0)
KayBee Posts: 241 Rating: (7)	Dear Expert, We have Step v 5.4 Sp4., OS XP Sp2 also tried in Sp3. We have misunderstanding that , inside FC or FB, We have some ladder logic to Start a Pump, and we want to latch the Pump start by Temporary variable. But the problem is in one FC it is working and in another not. Can we use temporary variable of FC or FB to latch a Coil?
KayBee Posts: 241 Rating: (7)
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4/16/2011 4:00 PM	Rate (0)
Darius Platinum Member Joined: 9/28/2005 Last visit: 8/22/2023 Posts: 2957 Rating: (242)	Hello, Well temporary variables for FC you can use only inside this FC. Once you finish executing FC all your temporary variables arfe lost. Other case is with FB. With every FB you have associated instance data block and all data (inputs, outputs, temp data and so on) are stored on that DB. So you can use that DB outside FB call. Thats the basics. If you want more comments then maybe put your porgram here and try to explain what really you need.
	Best regards, Darius
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4/16/2011 6:28 PM	Rate (0)
yy1 Gold Member Joined: 3/24/2009 Last visit: 6/20/2023 Posts: 535 Rating: (111)	Hello, I generally agreee with Darius, but I think, that temp-data isn't stored - neither in FC nor in FB. To store the data you have to use the stat-area. Best regards, yy1

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4/16/2011 8:27 PM	Rate (0)
Darius Platinum Member Joined: 9/28/2005 Last visit: 8/22/2023 Posts: 2957 Rating: (242)	Yes that's correct. Normally I use temp data for "real" temp application i.e. using only one time inside FB body. If I need reusable temporary variables I use stat area off course
	Best regards, Darius
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4/18/2011 7:46 AM	Rate (0)
KayBee Posts: 241 Rating: (7)	Dear Darius, Thanks for initiative. I have attached my test program here. I have very strange problem in OB1, you can see my description. Latching does work with Local variable when I dont call FB71in OB1 even when I call FB70, which also calling the FB71.Is not it strange. FB70 used START variable, FB71 uses Local variable, all FC using Local variable. I am not sure what is the exact secrete about S7 Local Variable in Fb and FC? Attachment Localvariable.zip (215 Downloads)
KayBee Posts: 241 Rating: (7)	Last edited by: PowerTags at: 4/18/2011 7:47 AM attachement.
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4/18/2011 9:12 AM	Rate (0)
Darius Platinum Member Joined: 9/28/2005 Last visit: 8/22/2023 Posts: 2957 Rating: (242)	If honestly I don't understand your program at all and it seems, that you don't understand what is LOCAL variable and what is STAT. Normally, if you use STAT in FB and this FB has lets say DB70 as its instance DB then you can access STAT variable inside this FB by accessing it as "#LatchIt". But if you want to access it from not withing FB then you should use syntax DB70.LatchIt. In case of FC you will not be able to access LOCAL variable outside function call and you will not be able to see what this variable contains. Its only LOCAL, means accessible only from FC.
	Best regards, Darius
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4/18/2011 9:37 AM	Rate (0)
Chilly_Garfield Posts: 316 Rating: (37)	Hi PowerTags, I think the problem will becoming with to much temp. variables in sum. If you overrun a certain limitin theoperation systemof the PLC, the PLCwill deallocate the memory. Then, I think, you get an error/fault in your program.
Chilly_Garfield Posts: 316 Rating: (37)
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4/18/2011 9:43 AM	Rate (0)
KayBee Posts: 241 Rating: (7)	Dear Darius, I know that much what you are saying.I understand what is STAT and Local memory in the FB or FC. But I have confusion over Local variable used to latch a coil. This Standard Block used in many Project previously as claimed by my friend. I only involved myself with this when see my friend is stugling over this FC/FB, when one FB [FB71 here] is bypassed logic is working when it is not bypassed it is not working. So what is that causes the problem? Please see the attachement in previous reply. [ the Program is to simulate the problem and concept] And for info. I also recomending my friend not to use Local Variable for Latching purpose and local variables are not useful once the block is executed and exited from the Block.
KayBee Posts: 241 Rating: (7)
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4/18/2011 11:00 AM	Rate (0)
yy1 Gold Member Joined: 3/24/2009 Last visit: 6/20/2023 Posts: 535 Rating: (111)	Dear PowerTags, in your project the '#LatchIt'-variable in FC70 and FB71 only work at random. If any other block is called between two calls to these blocks, the value of the variable is overwritten and not valid for further evaluation. So only the usage in FB70 is safe. Best regards, yy1

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4/18/2011 11:48 AM	Rate (0)
KayBee Posts: 241 Rating: (7)	Dear Yy1 I do agree to that. Is there any way to know how this RANDOM change in Local variable is happenning?
KayBee Posts: 241 Rating: (7)	Last edited by: PowerTags at: 4/18/2011 11:59 AM also how? what is that something "Secrete" operation change the content of Temporary variable? I have also gone through some FAQ/forum telling to initialize Local variable before first use in the Block. I agree with this also, as I had similar problem and problem solved after I do some initialize code in the beginning of Block. But here in case of Latching Coil by temporary variable, how do you want to initialize that Boolean temporary Flag [ LatchIt]. We can not initialize a Local Boolean temporary flag here because we are expecting that to be latched even after executing & exiting from the block. So actually it is "wrong" use of Local variable and while expecting "right" result. So the result of it is "Random". what is that secrete operation to make it random?
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