Issue
What purpose does Scan Interval serve on the point?
Product Line
TAC INET
Environment
All versions of I/NET software.
Cause
Understanding the Scan Interval on I/NET Points.
Resolution
The Scan Interval determines the length of time that elapses between scans. The controller scans the point based on the value set here (1-255 secs) and then updates the state/value on the screen.
Common Concerns for Choosing Scan Intervals.
- The Scan rate determines how often the point display is updated in either the controller summary or a graphics page. The fastest the page will update is based on the Monitor refresh interval set in the Host Configuration Editor.
- The Scan interval determines when the calculation on a point is run. This means if the point has a 10-second scan interval, the calculation will only be run every 10 seconds. It is advisable to only use calculations that do not require running 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. It will go old if the indirect point requests data from the global point and does not receive it after two scans. Global and indirect points should typically have higher scan rate values.
- The Scan Interval does not affect how often a global point broadcasts data. This is determined by the broadcast change in the counts setting or by a change of value for a discrete point.
- Higher values are usually better when dealing with scan intervals. This prevents hunting in DDC and the accumulation of old data.
- Whenever one point references another, these points should have identical scan rates. Problems have been reported on systems where the receiving point scans 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 is to use prime numbers for scan intervals on this type of point.
- For more information on Global and Indirect Points, see I/NET Global and Indirect Points Overview and Recommendations
