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(****************************************************************

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              RTC - Real-time clock

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Q is a copy of IN.

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When IN = FALSE, CDT is the current date and time as set by the

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PLC driver.

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When IN changes from FALSE to TRUE, PDT is stored. As long as IN is

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TRUE, CDT is equal to PDT + the amount of time since PDT was loaded.

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****************************************************************)

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FUNCTION_BLOCK RTC

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   VAR_INPUT

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     IN : BOOL;                  (* 0 - current time, 1 - load time from 

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PDT *)

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     PDT : DT;                   (* Preset datetime *)

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   END_VAR

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   VAR_OUTPUT

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     Q : BOOL := FALSE;          (* Copy of IN *)

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     CDT : DT;                   (* Datetime, current or relative to PDT *)

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   END_VAR

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   VAR

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     PREV_IN : BOOL := FALSE;

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     OFFSET : TIME;

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     CURRENT_TIME : DT;

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   END_VAR

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   {__SET_VAR(data__->,CURRENT_TIME,,__CURRENT_TIME)}

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   IF IN

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   THEN

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     IF NOT PREV_IN

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     THEN

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         OFFSET := PDT - CURRENT_TIME;

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     END_IF;

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     (* PDT + time since PDT was loaded *)

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     CDT := CURRENT_TIME + OFFSET;

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   ELSE

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     CDT := CURRENT_TIME;

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   END_IF;

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   Q := IN;

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   PREV_IN := IN;

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END_FUNCTION_BLOCK

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