Structured Text Programming
Rockwell Automation Publication 1756-RM006K-EN-P - November 2018 519
Structured text statements can also be instructions. A structured text instruction
executes each time it is scanned. A structured text instruction within a construct
executes every time the conditions of the construct are true. If the conditions of
the construct are false, the statements within the construct are not scanned. There
is no rung-condition or state transition that triggers execution.
This differs from function block instructions that use EnableIn to trigger
execution. Structured text instructions execute as if EnableIn is always set.
This also differs from ladder diagram instructions that use rung-condition-in to
trigger execution. Some ladder diagram instructions only execute when rung-
condition-in toggles from false to true. These are transitional ladder diagram
instructions. In structured text, instructions execute when they are scanned unless
pre-conditioning the execution of the structured text instruction.
For example, the ABL instruction is a transitional instruction in ladder diagram.
In this example, the ABL instruction only executes on a scan when tag_xic
transitions from cleared to set. The ABL instruction does not execute when
tag_xic stays set or when tag_xic clears.
In structured text, if writting this example as:
IF tag_xic THEN ABL(0,serial_control);
END_IF;
The ABL instruction will execute every scan that tag_xic is set, not just when
tag_xic transitions from cleared to set.
If you want the ABL instruction to execute only when tag_xic transitions from
cleared to set, you have to condition the structured text instruction. Use a one-
shot to trigger execution.
osri_1.InputBit := tag_xic;
OSRI(osri_1);
IF (osri_1.OutputBit) THEN
ABL(0,serial_control);
Components: Instructions
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