TON declared as PERSISTENT

2022-02-18T13:41:38.453000Z

I think I've answered my own question.

I believe that TON is using the TIME() function or something similar internally. If it does, the TON function would look something like this:

//TIME() function
//This function yields the time (in milliseconds) that has elapsed since system boot.

StartONS(CLK:=IN);
IF StartONS.Q THEN
    StartTime := TIME();
END_IF

IF NOT IN THEN
    StartTime:=TIME();
    Q:=FALSE;
END_IF

ET := TIME()-StartTime;        
IF ET>=PT THEN
    Q:=TRUE;
END_IF

Because the TON is stored in PERSISTENT RETAIN Memory, StartTime is retained through a power cycle.
The first time that the TON code is scanned after the PLC boots, TIME() will return a very small value, since it has just booted.

ET := TIME()-StartTime;

Results in an overflow causing ET to be large. When we get to this line of code, ET is greater than PT and Q is set TRUE.

IF ET>=PT THEN
    Q:=TRUE;
END_IF

TON declared as PERSISTENT

2022-02-17T20:08:56.218000Z

Can someone help me understand what is happening internally for a TON that is declared as PERSISTENT RETAIN?

VAR_GLOBAL PERSISTENT RETAIN
PersistTON: TON;
Enable: BOOL :=TRUE;
END_VAR

PROGRAM Main
VAR
Test: BOOL;
END_VAR

PersistTON(IN:= Enable, PT:=T#10M);
IF PersistTON.Q THEN
Test:=TRUE;
END_IF

If I power cycle the PLC when the timer is timing (well before ET=PT), PersistTON.Q is TRUE on the first scan and PersistTON.ET is set to PersistTON.PT.

If I power cycle when ET=T#30s, as soon as the logic is scanned again ET:=T#10M and Q is True.

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