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Conditionals |
  
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Conditionals |
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Conditionals supported and specified by IEC-61131-3
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NOTE!
Check if equipment supports conditionals!
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Conditional - IF
The IF statement allows a group of statements to be executed only if the associated boolean expression evaluates to true. If it evaluates to false, any subsequent conditions defined with ELSIF are checked. If any of these conditions are true, the corresponding statements within their THEN block are executed. If all conditions are false, the statements within the ELSE block, if present, are executed.
Therefore, at most only a single expression of the IF statement will be executed. The ELSIF and ELSE clauses are optional.
IF <boolean expression> THEN
<instructions>
ELSIF <condition> THEN
<instructions>
ELSE
<instructions>
END_IF
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Example
The example below demonstrates the logic for operating a cooling fan based on temperature conditions:
| • | If the temperature is below 25°C, the fan remains off. |
| • | If the temperature is between 25°C and 40°C, the fan operates at medium speed. |
| • | If the temperature is higher than 40°C, the fan operates at high speed and an alarm is activated. |
VAR alarm : BOOL; temperature : USINT; fanSpeed : USINT; END_VAR
IF temperature < 25 THEN fanSpeed := 0; alarm := FALSE; ELSIF temperature >= 25 AND temperature <= 40 THEN fanSpeed := 50; alarm := FALSE; ELSE fanSpeed := 100; alarm := TRUE; END_IF |
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Conditional - CASE
The CASE statement functions as a selector for groups of instructions, with each group identified by one or more literals, enumerated values, or subranges. When the selector's value matches a case, the corresponding group of statements is executed. If no match is found, the statements following the ELSE keyword (if present) are executed. Otherwise, no statements are executed.
The data types of these labels must match the data type of the variable specified in the selector.
CASE <expression> OF
<label_1>
<instructions>
<label_2, label_3>
<instructions>
<label_4..label_7>
<instructions>
ELSE
<instructions>
END_CASE
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Example
The example below illustrates logic in which an input value is determined by the position of an eight-position selector switch:
VAR selectorSwitch : USINT; input : USINT; END_VAR
CASE selectorSwitch OF 0..3: input := 25; 4: input := 35; 5, 6: input := 40; ELSE input := 50; END_CASE |
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