SLC 5/04 and daylight savings time

Ken Moore · Jun 22, 2004

I am currently working on a project that totalizes raw material charges on a 24 hour basis, each morning at 8am the current totals, are copied to storage registers, and then the totals are reset(zeroed). This is using an existing 1747-L542C and a RIO Panel View 1400.

The Panel view will display the current totals, and the stored data from yesterday.

My question is if this needs to happen every day at 8am, how do I handle day light savings time? I know I can access the PLC and change the time twice a year, but was hoping someone had already had to deal with this, and could give me a jump start. I'm not sure it's possible, since the time shift doesn't happen on a specific date, it is usually the first or last Sunday of the month.

thanks,

Ken

Beryl · Jun 22, 2004

The easiest way I know of is to synce the date/time registers to a SCADA PC that is doing the logging. The PC's automatically change for daylight savings. Hopefully you have this type of setup or something simmilar.

Ken Moore · Jun 22, 2004

No...SCADA isn't an option.
Modification of an existing system, with limited budget.

Ken

grs · Jun 22, 2004

A quick google gave me the daylight savings time changes up until 2009. You could probably find more and then just set up a subroutine in the SLC to update the system clock of the panelview and SLC when that exact date/time happens.

keithb · Jun 23, 2004

Here's a routine I used some years ago which worked OK on a 503

C5:0 is the day of week counter as the 503 I used had no day of week register.

B11:6/2 is used to prevent the clock continuously retarding 1 hour at 02:00

Note: this routine assumes UK daylight saving time changes on the last Sunday in March and the last Sunday in October.

Ken Moore · Jun 23, 2004

kiethb
Your solution looks good, what are you using to index the day of the week counter? I assume you looking at the hours and indexing at zero.

Ken

bernie_carlton · Jun 23, 2004

Other complications - This solution requires that the PLC is on at least during the hour of 2AM on each change day. How about routines which will work with the assumption that the PLC is on at least once during the Standard Time and during the Daylight Saving Time periods? This type routine could be called at PLC startup and then once at the start of each hour after that (OSR of seconds = 0). The testing eats up a lot of PLC time and this would mimimize that.

gbradley · Jun 23, 2004

Keith Cool logic!
Please explain.

B11:6/2 is used to prevent the clock continuously retarding 1 hour at 02:00

I assume the One shot B11:6/0 does this for March?
Why doesn't the One shot B11:6/1 do this in October?

In October you change the clock 30 seconds later. Why??

Thank-you

Ken Moore · Jun 23, 2004

Kieth,
I tried your logic, works great, had to change it for the US, which changes the first Sunday in April vs last one in March.

gbradley,

You need the latch because at 0200 you retard the clock to 0100,
you do not want to retard again at the second occurance of 0200. If you didn't have the latch , you would be stuck in a do loop. At 02 retard to 01, the when you get to 02, retard to 01 again.

Ken

gbradley · Jun 23, 2004

Thanks Ken

How does that go?
DoH

I know that this is Sacrilegious, but I've never been a fan of the Simpsons.

Ken Moore · Jun 23, 2004

I've uploaded my version of Kieth's logic.

keithb · Jun 24, 2004

gbradley

The 30 seconds later thing...
It was some years ago I wrote that logic, and I can't see any reason for it now.

gbradley · Jul 27, 2005

Ken
I was looking at the Counter for the Day of the week.
I learned from you last week that Counters only count on false to true transitions.
So why is the One shot needed in your example?

I tried it without the one shot, and it still works, but skips the 0.
I don't completely understand why.

Also I see that you had 0=Monday 6 = Sunday.

If I change the preset to 8 and use 1= Monday 7= Sunday everything seems to be ok.
What do you think?

rsdoran · Jul 27, 2005

The PLC will officially start at 00:00:00 but the transition from 24:59:59 will be hard to "see". I would assume the OSR is to prevent false triggering between "time counts"...ie if the second went to zero before the hour changed from 00 to 01. I think I ran into that once. The counter will count to Sun (7) and on next scan reset to zero so Mon will be 1. This gives you 7 counts to equal 7 days, 8 counts would add a day.

Greg Dake · Jul 27, 2005

One thing I never knew, It's "Daylight Saving Time" not "Savings". I realized this last year when we were doing 21CFR11 chart recorder stuff....time updates are a critical part of it.

Greg

SLC 5/04 and daylight savings time

Ken Moore

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Beryl

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Ken Moore

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grs

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keithb

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Ken Moore

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bernie_carlton

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gbradley

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Ken Moore

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gbradley

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Greg Dake

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