34 lines
1.8 KiB
Markdown
34 lines
1.8 KiB
Markdown
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#AT #Overlay_memory
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##### Overlaying tags with AT:
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##### Description
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***
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To access data areas within a declared tag, you can overlay the declared tags with an additional declaration. This provides you with the option of addressing an already declared tag with a different data type. You can, for example, address the individual bits of a tag of WORD data type with an ARRAY of BOOL.
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Alternatively, you can also use the instructions "SCATTER" and "GATHER" to break down bit sequences into an ARRAY of BOOL or group together individual bits to form a bit sequence. You can find these instructions in the "Basic instructions" task card in the "Move" area.
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##### Rules
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***
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The following general rules are valid for tag overlaying:
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Overlaying is possible in S7-1200 and S7-1500 in STL, LAD, FBD and GRAPH.
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SCL supports overlaying in all CPU families.
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Overlaying of tags is possible in the following blocks:
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In code blocks with standard access
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In code blocks with optimized access for tags with the retain setting "Set in IDB"
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The data width of the overlaying tag must be equal to or less than that of the overlaid tag.
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It is not possible to overlay tags of the VARIANT and INSTANCE data types.
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If the overlaid tag is of the data type STRING, ARRAY of BYTE, ARRAY of CHAR, etc., the length should correspond to an even number of bytes.
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Blocks from libraries which are declared as parameters in the interface cannot be overlaid.
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Structured PLC tags that are declared as parameters in the interface cannot be overlaid.
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You cannot address overlaying tags with slice access.
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![[Pasted image 20231017120328.png]]
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![[Pasted image 20231017124924.png]]
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To overlay a tag, declare an additional tag directly after the tag that is to be overlaid and identify it with the keyword "AT".
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In same cases is needed to disabled optimized block access
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![[Pasted image 20231017124228.png]]
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