Issue
What purpose does Scan Interval serve on the point?
Environment
All versions of I/NET software.
Cause
Points need to have their value updated on the controller.
Resolution
The Scan Interval is determines the length of time that elapses from one scan to another. The controller scans the point based on the value set here (1-255 secs) then updates the state/value on the screen.
Common Concerns for choosing scan intervals.
- The Scan rate determines how often point display is updated in either the controller summary or a graphics page. Keep in mind that the fastest the page will update is based on the Monitor refresh interval set in the Host Configuration Editor.
- The Scan interval determine when the calculation on a point is run. This means if the point has a 10 second scan interval, the calculation will only be calculated every 10 seconds. It is advisable to only use calculations that do not require being run faster than that as it can waste system resources. If you have a situation where logic requires an immediate response, use DDC or an event sequence.
- The Scan interval is used to determine when an indirect point goes OLD. Essentially, if the indirect point requests data from the global point and does not receive it after 2 scans, the point will go old. Global and indirect points should typically have slower scan rates.
- The Scan Interval does not affect how often a global point broadcasts data. This is determine by the broadcast change in counts setting or on a change of value for a discrete point.
- When dealing with Scan intervals, slower is usually better. This prevents hunting in DDC and old data from occurring.
- Whenever one point is referencing another, these points should have identical scan rates. Problems have been no to occur on systems where the receiving point is scanning faster than the sending point.
- When dealing with points that send or receive data from outside of the physical device. Consideration should be made to ensure all points on a given LAN segment are not broadcasting at the same time. The ideal solution to this is to use prime numbers for scan intervals on this type of point.