Logix5000 Controllers General Instructions - SLAC Confluence

Function Block Attributes 651Timing Modereal time samplingDescriptionIn the real time sampling mode, the delta time (DeltaT) used by the instruction is the difference between twotime stamp values that correspond to the updates of the process input. Use this mode when the process inputhas a time stamp associated with its updates and you need precise coordination.The time stamp value is read from the tag name entered for the RTSTimeStamp parameter of the instruction.Normally this tag name is a parameter on the input module associated with the process input.The instruction compares the configured RTSTime value (expected update period) against the calculatedDeltaT to determine if every update of the process input is being read by the instruction. If DeltaT is not within1 millisecond of the configuration time, the instruction sets the RTSMissed status bit to indicate that aproblem exists reading updates for the input on the module.Time-based instructions require a constant value for DeltaT in orderfor the control algorithm to properly calculate the process output. IfDeltaT varies, a discontinuity occurs in the process output. Theseverity of the discontinuity depends on the instruction and rangeover which DeltaT varies. A discontinuity occurs if the:• instruction is not executed during a scan.• instruction is executed multiple times during a task.• task is running and the task scan rate or the sample time of theprocess input changes.• user changes the time base mode while the task is running.• Order parameter is changed on a filter block while the task isrunning. Changing the Order parameter selects a differentcontrol algorithm within the instruction.Publication 1756-RM003I-EN-P - January 2007