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Saturday, 11 July 2015

I/O in an User Defined Type (UDT)

A User Defined Type (UDT) / structure is allocated as a contiguous block of memory. It Provides best performance for copy and message read/write operations. Because of this you cannot have an alias within a structure. While this would be nice from a design standpoint, it would have a detrimental impact on performance. Copy and Message operations would require significant overhead to detect the alias and perform the lookup of disjointed tag data.


If you need an I/O value in a UDT, define the UDT with a member to store the I/O value and buffer the I/O value into/out-of the UDT.
·     Make sure data-type of UDT member and I/O are the same type (avoids data type conversion)
·     Load input values into the UDT at the top of the program and copy output values out of the UDT at the bottom of the program
·     Single Bit – Use Examine on (XIC) and Output coil (OTE)
·     Contiguous bits – Use Bit Field Distribute (BTD) instruction
·     Single Value – Use Copy (COP) Instruction
·     Multiple Contiguous values – Utilize Synchronous Copy (CPS) instruction
·     Side benefit of providing synchronous I/O to program in that UDT, values will not change during the program scan

Q: How can you tell that a tag has not been used?
A: In tag editor select Show Unused tags to limit the view (filter) to tags that have not been referenced.


Q: How can I tell if a bit in a tag, element of an array, or member of a structure has not been used?
A: Run a cross-reference on root tag of the tag sort the list by reference or base tag and then look for gaps in the tags sequence.

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