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LPK

System clock accuracy

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Hi there,

 

Does anyone have any specifications on the accuracy of the real-time clock in the UniStream PLCs?

 

I am using one (a USP-070-B10) for an application that monitors a very slow-changing system, and so needs to be accurate (hopefully at least to the nearest second) over a number of consecutive days. Sometimes tests stretch into weeks.

 

I recently (about four weeks ago, but I’m not absolutely sure) noticed the clock was way off and synchronised it with my computer clock (which is accurate and auto-updates, etc.). This time I checked and it was between 4 and 5 minutes out, so seems like it might be deviating by about a minute a week. This is a real problem when we need to match up data on this system with data on other logging systems – it should not be more than a second or so out at any time if we are to do that accurately.

 

Also, is there any way to get the system to update with daylight savings time, so the clock does not need to be manually reset at these points?

 

Thank you for the advice.

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Hi,

 

The RTC error can be of 5 sec each couple of weeks.

The allowed drift is up tp 24 sec/month on a 25*C enviorment.

 

There is no direct way to automaticaly change the DST on the PLC via ladder logic.

 

Is the PLC connected to a network with acces to internet?

If it does, you can download from the next link examples for UniLogic.

http://www.unitronics.com/Downloads/Support/UniLogic/UniLogic%20Example%20Projects.zip

 

there you will find, under \Communications\Protocol a folder called RFC 1305 using message composer.

This example will show you how to to a logic on the UniStream so you can update the RTC via internet time server.

 

I hope this helps.

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Dealing with multiple clocks and drifts is always an issue.

 

When bringing multiple systems together for monitoring I highly suggest an intermediary application to request data from all of them simultaneously then store the results in a database.

 

The only other option is to have a sync setup of some sort to update the time between all of them continuously.

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