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FC "out" parameter with DBX

Created by: Goodas at: 3/5/2011 12:22 AM (2 Replies)

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3/5/2011 12:22 AM	Rate (0)
Goodas Posts: 37 Rating: (0)	Can you help? I have written an FC which is used many times. When the"out" parameters are assigned to either a 'temp' variable or a memory bit address every FC works as expected.However, whenevery 'out' parameter is assigned to an individual data block bit addressevery FC operates, when only the one being called should (as does with M address) Any explanation for this Thanks - Goodas.
Goodas Posts: 37 Rating: (0)
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3/5/2011 5:41 AM	Rate (0)
hdhosseini Diamond Expert Joined: 1/28/2009 Last visit: 2/21/2023 Posts: 6794 Rating: (1340)	Goodas I have written an FC which is used many times. When the"out" parameters are assigned to either a 'temp' variable or a memory bit address every FC works as expected.However, whenevery 'out' parameter is assigned to an individual data block bit addressevery FC operates, when only the one being called should (as does with M address) Please note that the legal interface of your program and called function is block parameters provided in each FC (at the top of LAD/FBD/STL editor).Using global data such as memories or data blocks as parameters can cause problem. You can pass data for processing to the called block and take over data from the called block.This data transfer is carried out via block parameters.CALL transfers the block parameter to the called block. BR
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3/5/2011 12:09 PM	Rate (0)
fritz Diamond Expert Joined: 10/7/2005 Last visit: 3/22/2023 Posts: 2962 Rating: (1022)	Hello Goodas I suspect that youare conditionally writing to the outputs (e.g. usage of "Set" or "Reset" or jumps over the instructions etc.) inside the FC which would explain what you observe. The"Golden rule" for an FC is that you MUST write to each output every time you call it. If this is not possible, you'llneed to use an IN_OUT or an make the FC an FB. You can find the offical Siemens explanation in the attached pic (which is taken from the inbuilt S7 help) and additonalbackground info on why the FC works correctly with non-DB addresses can be found in THIS thread. I hope this helps Attachment FC rules.jpg (78 Downloads)
	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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