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

(*
 * An IEC 61131-3 IL and ST compiler.
 *
 * Based on the
 * FINAL DRAFT - IEC 61131-3, 2nd Ed. (2001-12-10)
 *
 *)

 FUNCTION_BLOCK PID
   VAR_INPUT
     AUTO : BOOL ;        (* 0 - manual , 1 - automatic *)
     PV : REAL ;          (* Process variable *)
     SP : REAL ;          (* Set point *)
     X0 : REAL ;          (* Manual output adjustment - *)
                          (* Typically from transfer station *)
     KP : REAL ;          (* Proportionality constant *)
     TR : REAL ;          (* Reset time *)
     TD : REAL ;          (* Derivative time constant *)
     CYCLE : TIME ;       (* Sampling period *)
   END_VAR
   VAR_OUTPUT XOUT : REAL; END_VAR
   VAR ERROR : REAL ;        (* PV - SP *)
       ITERM : INTEGRAL ;    (* FB for integral term  *)
       DTERM : DERIVATIVE ;  (* FB for derivative term *)
   END_VAR
   ERROR := PV - SP ;
   (*** Adjust ITERM so that XOUT := X0 when AUTO = 0 ***)
   ITERM (RUN := AUTO, R1 := NOT AUTO, XIN := ERROR,
          X0 := TR * (X0 - ERROR), CYCLE := CYCLE) ;
   DTERM (RUN := AUTO, XIN := ERROR, CYCLE := CYCLE) ;
   XOUT := KP * (ERROR + ITERM.XOUT/TR + DTERM.XOUT*TD) ;
 END_FUNCTION_BLOCK