--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/lib/derivative_st.txt	Wed Jun 10 15:02:14 2009 +0200
@@ -0,0 +1,18 @@
+ FUNCTION_BLOCK DERIVATIVE
+   VAR_INPUT
+     RUN : BOOL ;          (* 0 = reset                  *)
+     XIN : REAL ;          (* Input to be differentiated *)
+     CYCLE : TIME ;        (* Sampling period            *)
+   END_VAR
+   VAR_OUTPUT
+     XOUT : REAL ;         (* Differentiated output      *)
+   END_VAR
+   VAR X1, X2, X3 : REAL ; END_VAR
+   IF RUN THEN
+      XOUT := (3.0 * (XIN - X3) + X1 - X2)
+              / (10.0 * TIME_TO_REAL(CYCLE)) ;
+      X3 := X2 ; X2 := X1 ; X1 := XIN ;
+   ELSE XOUT := 0.0; X1 := XIN ; X2 := XIN ; X3 := XIN ;
+   END_IF ;
+ END_FUNCTION_BLOCK
+