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AT Declaration dl-permalink__symbolPermalink

In the variables declaration, the code ‎AT‎ assigns a project variable to a specific input address, output address, or memory address of the PLC that is configured in the device tree. You can also define the assignment of variables to an address in the “I/O Mapping” dialog of the device in the PLC configuration.

Syntax 

<variable name> AT <address> : <data type>;

<address> : %<memory area prefix> ( <size prefix> )? <memory position>

The AT declaration allows you to give the address a meaningful name. You can make any necessary changes for the input or output signals at just one location, for example in the declaration.

Examples

‎VAR wInput AT %IW0 : WORD; END_VAR‎

Variable declaration with address information of an input word

‎VAR xActuator AT %QW0 : BOOL; END_VAR‎

Boolean variable declaration

Note: For Boolean variables, one byte is allocated internally if a single bit address is not specified. A change in the value of ‎xActuator‎ affects the range from ‎QX0.0‎ to ‎QX0.7‎.

‎VAR xSensor AT IX7.5 : BOOL; END_VAR‎

Boolean variable declaration with explicit specification of a single bit address

On access, only the input bit 7.5 is read.

‎VAR xSensor AT IX* : BOOL; END_VAR‎

For the address specification, the placeholder ‎*‎ is given instead of the memory position. The final address specification is done in the variable configuration.

Note: This is possible in function blocks.

If you assign a variable to an address, note the following:

  • You cannot write to variables that are placed at inputs. This will cause a compiler error.

  • You can perform AT declarations only for local and global variables, not for input/output variables of POUs.

  • Furthermore, AT declarations cannot be used in persistent variable lists.

  • If you use AT declarations for structure components or function block variables, then all instances use the same memory. This corresponds to the use of "static variables" in classic programming languages such as "C".

  • The memory layout of structures also depends on the target system.

NOTICE

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If you do not specify a single bit address explicitly, then Boolean variables are allocated byte-by-byte.

Example 

PROGRAM PLC_PRG
VAR
        xVar AT %QW0 : BOOL;
END_VAR

xVar := TRUE;

When the variable ‎xVar‎ is written, the output memory range from ‎QX0.0‎ to ‎QX0.7‎ is affected.