--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/lib/pid_st.txt	Wed Jun 10 15:02:14 2009 +0200
@@ -0,0 +1,25 @@
+ 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
+