Use duplicate symtable instead of symtable for enum constant value table (this will later allow us to detect semantic errors in IEC 61131-3 source code)
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