Applies To:
  • CitectSCADA 5.xx
  • CitectHMI 5.xx
Summary:
My trend history files are set to roll over at a specific time, but they do not seem to roll over at that time. The history files' time stamp shown in the Windows Explorer is one or more hours off, and the time varies from trend tag to trend tag, even though they have the same time and period set in the Trend Tag definition. 

Solution:
There are two reasons for this:

1. When you open the Trend Tags form in the Citect Project Editor and press F2 for extended options, you can set the trend time. This time is in the Greenwich Mean Time (GMT) time zone, not your local time zone. For example, Eastern USA standard time (EST) is GMT - 5 hours. During daylight savings time it is GMT - 4 hours. So, a trend set to roll over at 00:00:00 (midnight) will actually roll over at 7:00 PM (8:00 PM daylight savings time). This affects the beginning and end time of the data recorded in each history file.

2. The Citect trend server caches trend sample data to improve performance by reducing the amount of disk activity. The cache size, trend sample rate, and number of trend tags determine how long values can be cached before being written to disk. In Citect 5.31 and later there are 9 separate caches, each handling a different range of sample rates. The size of each cache is set by the [Trend]CacheSize<x> parameter (where x is 0 to 8). In Citect 5.30 and older there is only one cache. Its size is set by the [Trend]TrendBufSize parameter. Look up the parameter(s) appropriate to your Citect version in the Citect help for more information. The cache size affects the Windows created/modified timestamps on the trend data files, but does not affect the beginning and end times of the samples recorded in each file.

 
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