24 hour clock on a SLC 500 Fixed IO

JSPLC

Member
J
Join Date
Mar 2009
Location
Santa Rosa, CA
Posts
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I have a fixed IO SLC 500 that I need to program with a accurate 24 hour clock. The system does not have a date/time function so I used a free running TON to increment CTU's to provide minutes, hours and days. The accuracy is very poor and I was wondering if if anyone could suggest an alternate approach?
 
The thing about a TON is that it will usually over run the preset depending on the scan time of the other logic, this can be a significant error.

What I do for a long duration timer is to never reset the timer, instead, I subtract from the accumulator so that the overrun amount is still left there. I also always use the finest resolution timebase available.

Paul

timer_accuracy.jpg
 
Hey OkiePC,

I was using a very similar structure and I still had an accuracy issue. I actually varied the B source value of the GEQ to try and reach an accurate time over a few days but it still drifted. Part of me thinks that there could be a temerature stability issue that I am fighting (the unit is not in a controlled environment but not exposed to extreme temperature fluctuations).
 
Paul - I don't get the need for the 59 preset on the minute counter and 23 preset on the hour counter. The DN bit should go true when ACCUM >= PRESET.
 
bernie_carlton said:
Paul - I don't get the need for the 59 preset on the minute counter and 23 preset on the hour counter. The DN bit should go true when ACCUM >= PRESET.
Click to expand...

Yep...my mistake...That was not from a real program, just an example for the OP...

As for the accuracy, even with the GEQ---SUB, the clock itself is not extremely accurate.

At my previous job, we found that the SLC clocks would be off by up to two or three seconds per day and the PLC5s were even worse.

You may have to perform some sort of re-sync periodically to do better than that. Is the SLC connected to a network?
 
Well, how about something like one of these:
http://www.ipfonline.com/store/products/2287.html?catid=2287
http://www.omega.com/ppt/pptsc.asp?ref=PTC900
http://www.bb-elec.com/product_multi_family.asp?multifamilyid=39
http://literature.rockwellautomation.com/idc/groups/literature/documents/pp/1760-pp001_-en-p.pdf

In combination with Mildrones idea in Bernies link above...

Have the real time clock relay pull in every day at a predetermined time and cause an input to the SLC to turn on. Then in the SLC use that input to reset your clock registers.

Paul
 
I have often thought that an external trigger would be an easy solution, and I just might go that route, though I was hopping to be able to solve it with just the PLC.

PS - I just want to thanks everyone for their input - this is a great site and a great community.
 

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