1  FUNCTION_BLOCK PID

     2    VAR_INPUT

     3      AUTO : BOOL ;        (* 0 - manual , 1 - automatic *)

     4      PV : REAL ;          (* Process variable *)

     5      SP : REAL ;          (* Set point *)

     6      X0 : REAL ;          (* Manual output adjustment - *)

     7                           (* Typically from transfer station *)

     8      KP : REAL ;          (* Proportionality constant *)

     9      TR : REAL ;          (* Reset time *)

    10      TD : REAL ;          (* Derivative time constant *)

    11      CYCLE : TIME ;       (* Sampling period *)

    12    END_VAR

    13    VAR_OUTPUT XOUT : REAL; END_VAR

    14    VAR ERROR : REAL ;        (* PV - SP *)

    15        ITERM : INTEGRAL ;    (* FB for integral term  *)

    16        DTERM : DERIVATIVE ;  (* FB for derivative term *)

    17    END_VAR

    18    ERROR := PV - SP ;

    19    (*** Adjust ITERM so that XOUT := X0 when AUTO = 0 ***)

    20    ITERM (RUN := AUTO, R1 := NOT AUTO, XIN := ERROR,

    21           X0 := TR * (X0 - ERROR), CYCLE := CYCLE) ;

    22    DTERM (RUN := AUTO, XIN := ERROR, CYCLE := CYCLE) ;

    23    XOUT := KP * (ERROR + ITERM.XOUT/TR + DTERM.XOUT*TD) ;

    24  END_FUNCTION_BLOCK

    25