Safety Considerations Guide, Tricon v9.0 - Tuv-fs.com

Setting Scan Time 53Setting Scan TimeSetting appropriate scan time for the project is essential to avoid impropercontroller behavior. When changing a project running in an on-line system, specialprecautions should be exercised to avoid scan time overruns, which could result inunexpected controller behavior.Scan TimeScan time is the interval required for evaluations (in other words, scans) of anapplication as it executes in the controller. The time it actually takes to do anevaluation may be less than the requested scan time. To prevent scan-timeoverruns, a scan time must be set that includes sufficient time for all executableelements in an application—including print statements, conditional statements,and future download changes.Use the Scan Time parameter in the TriStation Configuration editor to suggest thedesired scan time before downloading an application. Upon downloading, thecontroller determines the minimum and maximum allowable scan times for yourapplication and uses your suggested scan time if it falls within the acceptablelimits. The default scan time is 200 milliseconds. The maximum allowable scantime is 500 milliseconds and the minimum allowable scan time is 25 milliseconds.Requested Scan Time is a system parameter that is set using the Configurationeditor.Actual scan time is the actual time of the last scan. Actual scan time is always equalto or greater than the requested scan time.Scan SurplusScan surplus is the scan time remaining after application elements have beenexecuted. Scan Surplus must be positive—if it is negative, the Scan Timeparameter must be adjusted using the Set Scan Time command on the ControlPanel to set the surplus value to greater than or equal to zero. The Scan Timeparameter in the TriStation Configuration editor applies only when you do aDownload All.Chapter 4Application Development