📖HomeBack The compression which is available on the historic tab for a point is applied by the database once the data is passed from the driver to the database. In its simplest form it is designed for outstations / PLCs whose points only have current data and do not provide anyway within the outstation to log data or filter the updates. The prime example of when this would be used would be the Modbus driver that is scanning a point at a fast interval and storing data historically. The compression on the historic would then be used to limit the amount of data ending up in the historic database.
For most Advanced Drivers that perform time-stamped logging, it is recommended that the compression on the historic tab is not turned on. Instead, the filtering is generally provided by the outstation itself so that it only logs a value when it exceeds some threshold.
The compression algorithm is applied to just the value of the point. If the state or reason for logging changes, then the value will always be logged in the historic database. This would explain why a small change such as 0 to 0.01 is logged in the database. This would most likely be a change from the Low Low to Normal states and would be logged because of this state change, irrespective of the fact that the compression deadband had not been exceeded.
For the DNP3 driver, when the reason changes from Value Change (event data) to Current (static data) or vice versa, the value will always be stored historically. The Historic Data Filter on the point can be configured as Static and Event Data, which means the DNP3 driver will send both static (current) and event (value change) data to the historian. If you set this to just Event Data the static (current) data will not be sent.
For simple drivers such as Modbus, avoiding small changes being written to the historian on state change eg. Low Low to Normal when 0 to 0.01, can be performed by using the Significant Change setting on the Modbus tab of the point configuration. The Significant Change determines how much the point value will need to change before it is passed to the database. The value would then still be logged on state change, but the state change would not occur until the data exceeded the significant change setting and was then passed to the database for processing. This setting will filter out jitter in the input so that it is not processed at all. It is normal practice to configure a significant change deadband on almost all analogue points to prevent this type of issue. By default Modbus analog points have a 1% deadband.