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• TOF_R / TOF_R_LTIME

TOF_R / TOF_R_LTIME

Type	Function block
Description	The TOF_R / TOF_R_LTIME function blocks provide a timer for delayed switching off. They operate as retain timers, i.e., the elapsed time since start of the timer is retained internally when switching the controller off. After switching the controller on again and executing a warm start, the timer continues counting from the point at which it was stopped. If IN changes from TRUE to FALSE, switching off is delayed for the duration of the time interval applied to the input PT. After PT has elapsed, Q is set to FALSE. The elapsed time interval is indicated at the output ET.
Notes	The input IN and the output Q can be negated. Note: The negation of formal parameters is not supported in safety-related code (SNOLD). Function blocks have to be instantiated. The instance name of the function block has to be declared in the 'Variables' table of the POU where the FB is going to be used. The instance name must be unique within the POU.
Parameters	Inputs IN Data type: BOOL Description: If a falling edge is detected, the off-delay timing is started. Changing the input from FALSE to TRUE while the timer is running, stops and resets the timer (ET = 0). PT Data type: TOF_R: TIME TOF_R_LTIME: LTIME Description: Preset time interval for the delay. The time value must be positive. Outputs Q Data type: BOOL Description: TRUE if IN = TRUE and ET < PT. FALSE if IN = FALSE and ET >= PT. ET Data type: TOF_R: TIME TOF_R_LTIME: LTIME Description: Elapsed time interval (retentive). The current elapsed time is retained internally when switching the controller off. After switching the controller on again and executing a warm start, the timer continues counting from the point at which it was stopped.
Timing Diagram	Timing diagram 0 The IN input is FALSE. As a result, Q is also set to FALSE. 1 IN switches from FALSE to TRUE. Q is set to TRUE immediately. 2 IN switches from TRUE to FALSE. The measurement of the elapsed time is started. 3 Power is switched off. The timer stops. The elapsed time since start of the timer is retained. 4 Power is switched on again and a warm start is executed. The timer continues counting from the point at which it was stopped. 5 When the delay time PT has elapsed (ET = PT), Q is reset to FALSE. 6 IN switches from FALSE to TRUE. Q is set to TRUE immediately and the timer is reset (ET = 0) 7 IN switches from TRUE to FALSE again. The measurement of the elapsed time is started once more. 8 When the delay time PT has elapsed (ET = PT), Q is reset to FALSE.
Algorithm	Algorithm The algorithm is based on incrementing time distances between two invocations of the function block. The function block is capable to calculate the elapsed time after a warm restart beginning from the last invocation. The time distance is measured by using an internal system tick counter (contains the amount of time in milliseconds). The preset timer value (PT) is limited at 2,147,483,647 milliseconds for the *_R function block (max range of DINT data type) and 9,223,372,036,854,775,807 for the *_R_LTIME function block (max. range of the LINT data type). Problem of lost invocation after warm restart Because the algorithm is based on a non retentive internal system tick counter, it is important to take notice of the problem, which we call 'lost invocation'. The figure below describes the behavior of the function block before and after a warm start is forced. After the parameter IN raised to TRUE, the next invocation detects the raising edge state of IN. At this point, the calculation begins with ET (elapsed time) := 0. After a warm restart, the next invocation detects the changed state and normalizes the internal start value by subtracting the current system counter with the internal retentive elapsed counter. That is why the time distance to the previous invocation is 0. Related to the 'real' elapsed time there is a delay dependent from the duration between successive invocations of the function block. That means, after a warm start the duration of the function block is at worst one invocation distance slower than a function block where no warm restart occurs. Hot start behavior After a hot start is performed , it is important to know that the timer calculates the elapsed time including the duration of the stop state. A hot start is forced, if, for example, the PLC is continued from a breakpoint. During the online debug, it could happen that some timer expire if the PLC halts on breakpoints.