--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/lib/rtc.txt	Sat Mar 31 15:34:33 2012 +0100
@@ -0,0 +1,51 @@
+(****************************************************************
+
+              RTC - Real-time clock
+
+Q is a copy of IN.
+
+When IN = FALSE, CDT is the current date and time as set by the
+PLC driver.
+
+When IN changes from FALSE to TRUE, PDT is stored. As long as IN is
+TRUE, CDT is equal to PDT + the amount of time since PDT was loaded.
+
+****************************************************************)
+
+FUNCTION_BLOCK RTC
+   VAR_INPUT
+     IN : BOOL;                  (* 0 - current time, 1 - load time from 
+PDT *)
+     PDT : DT;                   (* Preset datetime *)
+   END_VAR
+   VAR_OUTPUT
+     Q : BOOL := FALSE;          (* Copy of IN *)
+     CDT : DT;                   (* Datetime, current or relative to PDT *)
+   END_VAR
+
+   VAR
+     PREV_IN : BOOL := FALSE;
+     OFFSET : TIME;
+     CURRENT_TIME : DT;
+   END_VAR
+
+   {__SET_VAR(data__->,CURRENT_TIME,__CURRENT_TIME)}
+
+   IF IN
+   THEN
+     IF NOT PREV_IN
+     THEN
+         OFFSET := PDT - CURRENT_TIME;
+     END_IF;
+
+     (* PDT + time since PDT was loaded *)
+     CDT := CURRENT_TIME + OFFSET;
+   ELSE
+     CDT := CURRENT_TIME;
+   END_IF;
+
+   Q := IN;
+   PREV_IN := IN;
+
+END_FUNCTION_BLOCK
+