Allen-Bradley 1756 ControlLogix - Structured Text Components: Constructs

Overview of Structured Text Programming
Chapter 11
Rockwell Automation Publication MOTION-RM002H-EN-P-February 2018 645
from cleared to set. The ABL instruction does not execute when tag_xic stays set
or when tag_xic is cleared.
In structured text, if you write 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);
END_IF;
Constructs can be programmed alone or nested within other constructs.
If you want to Use this construct
Do something if or when specific conditions occur IF. . . THEN
Select what to do based on a numerical value CASE. . . OF
Do something a specific number of times before doing anything else FOR. . . DO
Keep doing something as long as certain conditions are true WHILE. . . DO
Keep doing something until a condition is true REPEAT. . . UNTIL
Structured Text
Components: Constructs