Structured Text Programming
Rockwell Automation Publication 1756-RM006K-EN-P - November 2018 511
The tag retains the assigned value until another assignment changes the value.
The expression can be simple, such as an immediate value or another tag name, or
the expression can be complex and include several operators and functions, or
both. Refer to Expressions for more information.
Tip:
I/O module data updates asynchronously to the execution of logic. If you reference an input multiple times in your
logic, the input could change state between separate references. If you need the input to have the same state for
each reference, buffer the input value and reference that buffer tag. For more information, see
Logix 5000
Controllers Common Procedures , publication 1756-PM001 .
You can also use Input and Output program parameters which automatically buffer the data during logix
execution. See LOGIX 5000 Controllers Program Parameters Programming Manual , publication 1756-PM021
.
See also
Assign an ASCII character to a string data member on page 512
Specify a non-retentive assignment on page 511
Structured Text Components: Expressions on page 513
Character string literals on page 521
The non-retentive assignment is different from the regular assignment described
above in that the tag in a non-retentive assignment is reset to zero each time the
controller:
• Enters the Run mode
• Leaves the step of an SFC if you configure the SFC for Automatic reset.
This applies only if you embed the assignment in the action of the step or
use the action to call a structured text routine by using a JSR instruction.
A non-retentive assignment has this syntax:
tag [:=] expression ;
where:
assignment
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