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Local variable remain SET even after power off-on.

9/24/2012 8:38 AM	Rate (0)
P.Singhania Posts: 12 Rating: (0)	Dear Experts, I recently faced a problem regarding local variable. A local variable was defined (say Bit X) in In-Out area and was set high if the Profibus instructions SFC14 & SFC15 RET_VAL moves away from Exact ' 0'. Means the bit-X is set when the communication with the DP slave breakes. The Dp slave had a problem & was changed. Now the Bit-X was set to ' 1 ', as Dp slave gone faulty. Now even if the RET_VAL is ' 0 ' , and the slave is communicating, the BIT-X is permenently set to ' 1 ' . Even after power off-on of the CPU several times. The problem is resolved by resetting the bit-X by writing logic. The CPU is 314C-2DP. So, u pls explain why dose this happened because I have no proper justification to give to the customer.
P.Singhania Posts: 12 Rating: (0)
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9/24/2012 11:30 AM	Rate (0)
fritz Diamond Expert Joined: 10/7/2005 Last visit: 9/14/2025 Posts: 3049 Rating: (1062)	Hello P.Singhania by "local variable" I presume you mean a TEMP variable of an FC or FB, correct? If so and ifindeed used as an IN_OUT variable you may want to check if an unconditional write access happens to it before a read access. If not, you violate the rule for usage of TEMP variables which are only valid while the Block is executed and as such must be initialised (i.e. do an unconditional write to them before a read access). I hope this helps
	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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9/24/2012 11:36 AM	Rate (0)
P.Singhania Posts: 12 Rating: (0)	Thanks fritz for replying, Yes, I mean temp variable from FB. Well, the same I have done what you said, but eventually it is a system fault. Right??
P.Singhania Posts: 12 Rating: (0)
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9/24/2012 2:20 PM	Rate (0)
fritz Diamond Expert Joined: 10/7/2005 Last visit: 9/14/2025 Posts: 3049 Rating: (1062)	P.Singhania ...but eventually it is a system fault. Right?? I'm afraid to say it is NOTa system fault,but rather fault by whoever programmend the logic. TEMP variables simply do NOT retain their value between Block calls. They are - as the name suggests - temporary andcancan assume any value after the Block that used them has finished executing. As mentioned before, the simply rule is to (unconditionally write to them inside a Blockbefore you read from them (the first 20 Bytes of an OB are the exception to this and written to by the OS before the OB is called). I hope this helps
	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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