In time-based processing, SAC processes a block at a set time. The following table lists the scan time ranges you can enter for time-based chains.
SAC scans chains with hour and minute scan times based on the system clock of the local SCADA server. Scan times are set relative to midnight (00:00:00 hours). SAC scans chains with second and subsecond scan times based on the computer's start up time, as the following tables describes.
Assigning Time-Based Scan Times
You can assign a time-based scan time to a block by completing its Scan Time field with the following format:
time unit
The following table lists the valid units and their abbreviations. If you do not enter a unit of time, iFIX assumes the unit is seconds.
|
Unit |
Entry |
|
Minutes |
M |
|
Hours |
H |
Example: Assigning Time-Based Scan Times
To scan a block every 3 hours, enter:
3H
Because SAC processes this scan time based to the system clock, it scans the block at 0:00, 3:00, 6:00, 9:00, and 12:00 regardless of when you place it on scan.
Follow these guidelines when you assign scan times:
- Assign scan times larger than the poll rate assigned in the I/O driver. This ensures that the I/O driver has time to read and send new values to SAC before SAC scans each block again. See your I/O driver manual for more information about the poll rate.
- Phase (stagger) scan times to reduce the risk of overloading the CPU. Refer to the section Phasing to learn more about phasing.
- Assign critical process chains a more frequent scan time than non-critical chains. If you need to have a chain scanned every 2 minutes, assign a 2-minute scan time, not a 5-second scan time. Remember that very short scan times require more CPU time and SAC processing than longer scan times.
- If a chain does not need processing at a set time, assign exception-based processing. Doing so will require less CPU time and improve performance.
