AT Declaration

Using a clear configuration editor for I/O mapping

We advise against assigning memory addresses as described here, because this can lead to a loss of clarity.

We recommend assigning memory addresses to variables in the configuration editor Edit IO mapping of the device.

The AT keyword in the variables declaration assigns to a project variable a specific input address, output address, or memory address of the controller which is configured in the device tree.

The AT declaration makes it possible to give an address a meaningful name.

Tip

You can make any necessary changes for the input or output signals at just one location (for example, in the declaration).

For more information, see:

Syntax

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

Required

`%` <address>	Required Address in the memory area `%` <memory area prefix> <size prefix> <memory position>
<memory area prefix>	Required `I`: Input memory area Memory area for input signals and inputs For physical inputs via input drivers and sensors `Q`: Output memory area Memory area for output signals and outputs For physical outputs via output drivers and actuators `M`: Flag memory area
<size prefix>	Optional No size prefix: bit Single bit `X`: Bit Single bit `B`: Byte 8-bit data width `W`: Word 16-bit data width `D`: Dword 32-bit data width
<memory position>	<position number> `.` <bit position number> Example: `%IB2.4` stands for the 5th bit of the 3rd byte
<position number>	Required The numbering depends on the target system and starts at 0.
`.` <bit position number>	Optional 0..7 for one byte

Optional

The data type should match the size prefix.

Example 21. 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: In the case of Boolean variables, one byte is allocated internally when 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 When accessed, 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.

Helpful Hints

Important

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

Important

Individual bit addresses which are mapped to Boolean variables have restrictions. It is not possible to use these variables with VAR_IN_OUT, REFERENCE TO, or the ADR operator.

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

You cannot write to variables which are placed at inputs. This will cause a compiler error.
You can make AT declarations only for local and global variables, not for input and output variables of POUs.
You must not use AT declarations in persistent variable lists.
If you use AT declarations for structure members 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.

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.