Why are there gaps in trend?

FAQ #100938

 

Symptom
It is possible that "holes" in "HDLog to DB" trends are visible in the browser if the "load info & data" of the macro "Trend" is executed after a longer time interval than the data memory size available in the PCD allows to store!


Reason
The trend data's loaded from the PCD are stored temporarily on the memory of the Web browser.
The gaps can occur if the displayed time range on the browser is bigger than the data logging ringbuffer on the PCD.

Example
The log date size is 100 KByte data with a frequency of 1 log per seconds.
As 1 KB is 1024 bytes, there are 102400 bytes (100 x 1024 bytes) available for the trend.
1 log entry including time stamp (date/hour/value) takes 8 bytes.
As result, the available DBs on the PCD are filled up after 12800 seconds (102400 bytes / 8 bytes) which corresponds to 3h and 33 minutes. 

If the "load info & data" is executed at at 8.00 PM and again at 12.00 PM, a gap of 26.66 minutes will be displayed between the two ranges of logs.
At 8.00 PM the data's from 4.26.66 PM to 8.00 PM are loaded from the PCD, stored in the temporary memory of the browser and displayed on the browser.
At 12.00 PM, the data's from 8.26.66 PM to 12.00 PM are loaded from the PCD, added to the data's loaded before at 8.00 PM and stored in the temporary memory of the browser.
All data's (from 08.00 PM and 12.00 PM) are then displayed on the browser.
No recorded data is available between 8 and 8.26.66 PM because the data recorded in this period has been overwritten by the ringbuffer on the PCD.

Note
If the browser is restarted or if a "clear Log" is exectuted then the gap disappaers because the temporary memory of the browser is cleared and trend shows only the last data's (3h33.33) uploaded from the PCD!