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Limits
Technological limits can be defined only for some historical valuesare effective only for historical values which are calculated by D2000 Archiv (evaluated and statistical historical values). There are 4 limits defined: VHL, HL, LL and VLL. Limit can be defined either directly – by the value entry into the input field, or its value can be determined by a system object (dynamic limit) – the button right from the input field.
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Optimisation of old values processing coming from communication (automatically or as a result of TELL command GETOLDVAL) or values of remote tags (as a result of TELL command GETOLDVAL).
If the checkbox is checked, during processing of old value the archive database is queried whether the value is already present there. If it exists, the value is discarded (and recalcs of statistical or calculated archived values which use this primary archive value are not performed either).
Note: Optimisation is useful e.g. for archiving of I/O tags from communication using the protocol IEC62056-21:2002 File I/O. Its communication files contain several historical values (which KOM process sends as old values) and one new value for every I/O tag.
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- Continuous - continuous (on the fly) calculation. Result values are calculated on the fly and they are automatically available (in dependence on the system load). A disadvantage of the method is a higher demand on the computing power (especially for frequent changes of primary historical values).
- On demand - calculation is executed and result is stored to the archive on demand. The demand can be generated by the action CALCONDEMANDSTAT or the Tell command RECALC).
Note: historical value calculated on demand should not have any depending historical values calculated continuously, because the result would be wrong. - On read - calculation is executed as a result of a read request. An advantage of this method is that values are not stored in the database so they don't occupy any disk space. There is no possibility of re-calculation in case of writing delayed data into the archive. A disadvantage is a necessity to read source data and calculate it for each read demand.
Note: historical value calculated on read should not have any depending historical values calculated continuously or on demand, because the result could be wrong (due to delayed data) or the calculation could be ineffective (multiple calculations of a single on read object if it is used by several other historical values).
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- Continuous - continuous (on the fly) calculation. Result values are calculated on the fly and they are automatically available (in dependence on the system load). A disadvantage of the method is a higher demand on the computing power (especially for frequent changes of primary historical values).
- On demand - calculation is executed and result is stored to the archive on demand. The demand can be generated by the action CALCONDEMANDSTAT or the Tell command RECALC). Note: historical value calculated on demand should not have any depending historical values calculated continuously, because the result would be wrong.
- On read - calculation is executed as a result of a read request. An advantage of the method is that values are not stored in the database so they don't occupy any disk space. There is no chance of wrong calculation in case of writing delayed data into the archive. A disadvantage is a necessity to read source data and calculate it for each read demand.
Note: historical value calculated on read should not have any depending historical values calculated continuously or on demand, because the result could be wrong (due to delayed data) or the calculation could be ineffective (multiple calculations of a single on read object if it is used by several other historical values).
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