timed loop drift

[lincolnven]

thanks for the suggestions David T

At present, I am noting the 3s deficit from the timestamps logged to the datalog files. In this particular loop, there are not really any graphs or indicators, so the only real problem may be the file I/O, or that other loops with many indicators are consuming too much processor power. Would you recommend making this loop into a time critical sub-VI? This is the only loop that really needs to be deterministic...

Could you also provide som pointers to RT programming tips. I have found that the manual with the RT module only really deals with simple structures, not nested ones.

Thanks

David Williamson 

[DavidT]

Hi,

Did you get to try out some of the suggestions? Is there any difference in timestamps when using a 'tick count' vi instead of the 'get time/date'? I would recommend making your deterministic vi, time critical but im not sure if this would have any impact in your application since you have such long wait periods for all your loops.

The link below has recommendations for building applications for FieldPoint. Have a bit more of a look through the NI site for tips on developing RT applications. I would also avoid using File Open and Close vis, as well as avoiding large numbers of controls and indicators.

http://zone.ni.com/devzone/conceptd.nsf/webmain/60644843d0c0395286256d17007ff5fd

Hope this helps.

[lincolnven]

Hello David T,

sorry for taking so long to reply, I only work this job 1 day a week and the fieldpoint is some 200km away...

Yesterday I finally had the opportunity to try some of your suggestions. It turns out that, as Jeremy Taylor from NI suggested, the timing problems with my loop are related to the millisecond timer. The millisecond timer gives 1 tick every 1.000686ms. When timing 15 minutes (900000ms) the difference between ticks and and real ms is 617 ticks.

However, this is only 0.617s, not the 3 seconds I noted. This difference was corrected by reading a timestamp instead of the Get Date/Time vi, as you suggested. I noticed that in the five weeks since I ran the RT Set Date/Time vi, the Get Date/Time output is about 2 hours(!) behind the timestamp output, the latter of which is still perfectly synchronised to the laptop. 

Any idea why???

Regards

David Williamson