Understanding Refresh Rates for Enhanced Charts
There are a number of refresh rates that are involved with presenting data on a chart. In order to get the best appearance from your charts, it is necessary to understand how the rates interact.
|
Type of Refresh Rate |
Definition |
|
Block Scan Time |
The scan time, or the frequency of the interaction between the PLC and the SCADA. |
|
Real-time Data Refresh Rate |
The frequency at which the database updates the information from the block. This rate should not be set to a higher value than the block scan time value. This value is set on the Expression Builder dialog box. |
|
Chart Update Rate |
The frequency at which the chart retrieves data from the database. This rate should not be set to a higher value than the real-time data refresh rate. Doing so may result in flat line areas in your chart, because the chart is retrieving data faster than the database is receiving updates. This value is set in the Appearance area of the General tab of the Chart Customization dialog box. For XY charts, this update rate applies to both real-time and historical data sources. |
|
Historical Update Rate |
The frequency at which the chart retrieves historical data from the Historian. This value is completely independent of the other three refresh rates, which are interdependent. Because the historical update rate is independent of the other refresh rates, when historical data is plotted on a chart with real-time data, it will not update in the same way. Its plotting behavior is best exemplified on Standard Charts. NOTE: This option does not apply to XY charts. |
