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(* Following taken directly from the IEC 61131.3 draft standard *)

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(*

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 * An IEC 61131-3 IL and ST compiler.

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 *

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 * Based on the

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 * FINAL DRAFT - IEC 61131-3, 2nd Ed. (2001-12-10)

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 *

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 *)

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(*

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 * This is part of the library conatining the functions

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 * and function blocks defined in the standard.

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 *

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 * Counter function blocks

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 * -----------------------

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 *)

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FUNCTION_BLOCK CTU

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  VAR_INPUT

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    CU : BOOL;

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    R : BOOL;

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    PV : INT;

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  END_VAR

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  VAR_OUTPUT

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    Q : BOOL;

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    CV : INT;

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  END_VAR

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  VAR RETAIN

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    PVmax : INT;

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  END_VAR

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  IF R THEN CV := 0 ;

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  ELSIF CU AND (CV < PVmax)

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       THEN CV := CV+1;

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  END_IF ;

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  Q := (CV >= PV) ;

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END_FUNCTION_BLOCK

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FUNCTION_BLOCK CTD

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  VAR_INPUT

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    CD : BOOL;

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    LD : BOOL;

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    PV : INT;

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  END_VAR

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  VAR_OUTPUT

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    Q : BOOL;

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    CV : INT;

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  END_VAR

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  VAR RETAIN

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    PVmin : INT;

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  END_VAR

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  IF LD THEN CV := PV ;

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  ELSIF CD AND (CV > PVmin)

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      THEN CV := CV-1;

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  END_IF ;

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  Q := (CV <= 0) ;

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END_FUNCTION_BLOCK

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FUNCTION_BLOCK CTUD

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  VAR_INPUT

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    CU : BOOL;

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    CD : BOOL;

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    R  : BOOL;

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    LD : BOOL;

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    PV : INT;

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  END_VAR

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  VAR_OUTPUT

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    QU : BOOL;

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    QD : BOOL;

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    CV : INT;

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  END_VAR

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  VAR RETAIN

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    PVmax : INT;

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    PVmin : INT;

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  END_VAR

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  IF R THEN CV := 0 ;

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  ELSIF LD THEN CV := PV ;

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  ELSE

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    IF NOT (CU AND CD) THEN

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      IF CU AND (CV < PVmax)

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      THEN CV := CV+1;

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      ELSIF CD AND (CV > PVmin)

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      THEN CV := CV-1;

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      END_IF;

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    END_IF;

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  END_IF ;

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  QU := (CV >= PV) ;

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  QD := (CV <= 0) ;

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END_FUNCTION_BLOCK

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