Additional options that affect the swinging door deadband

The swinging door deadband (the rate deadband) can optionally be combined with a value deadband and/or a deadband override period. This combination will affect which values are actually stored.

If the data is store-and-forward data from a remote IDAS, the behavior of the swinging door algorithm also depends on the value of the real-time window in the AVEVA Historian, as specified by the RealTimeWindow system parameter.

• Value deadband
  When combined with rate deadband (with or without a deadband override period), the value deadband is always applied first, followed by the other deadbands. For the value deadband, the system checks the difference in value between the received point from the value of the last stored point. Only when this difference exceeds the value deadband does the system consider the point for rate evaluation.

• Deadband "override" period
  If the elapsed time since the last stored point exceeds the deadband override period, the last received point before the time at which the deadband override period expired is stored, regardless of value and rate deadband.

• Real-time window (Classic Storage subsystem)
  The real-time window setting for IDAS store-and-forward data (RealTimeWindow system parameter) allows for the expansion of the time window for which the storage system considers data to be "real-time." The real-time window is important for swinging door deadbanding because it determines the maximum length of time that a point will be "held" by the storage system without storing it, while waiting for the next point. For more information, see About the real-time data window.

Real-time window and deadband override periods are two independent modifiers that force the storage of received points that may have otherwise been discarded due to the setting of either the rate deadband or the value deadband.

The real-time window specification is more likely to select points for storage when the time period between points received from the source is less than the real-time window, but the slope of the incoming data values is such that the rate deadband excludes the points from being stored.

The deadband override period is more likely to select points for storage if the rate at which points are received from the data source is slow (slower than the real-time window) and the rate deadband excludes the points from being stored.

For an illustration of how these factors work together to determine the actual values to be stored, see Swinging door deadband examples.

Whatever the combination of rate deadband, value deadband, and deadband override period specified, only points actually received from the data source are stored on disk. That is, points to be stored on disk are never "manufactured" by the swinging door algorithm. This is particularly relevant in understanding the behavior implied by specifying the real-time window and the deadband override period.