Using Concurrent I/O to Count Time between Events

This article explains a possible way to determine the amount of time taken between the start and end of an event. The event doesn't have to start and end within the same job using this method. 

The following example tracks the time it takes to complete a task using an Output which is turned ON at the beginning of the task and turned OFF when the task is completed.

#15057 = Cycle Time Output

M250 = Cycle Time

M251 = Timer Set Value (units of 0.1 seconds)

M252 = Timer Current Value

Breakdown of above logic:

When Output (#15057) turns ON, the Timer Current Value (M252) will count down from the Timer Set Value (M251). When Output (#15057) turns OFF, Auxiliary Relay (#76001) will be pulsed for one ladder scan. In this ladder scan, the Timer Current Value (M252) is outputted to Cycle Time Complete register (M250). Then the Cycle Time Complete (M250) is set to Timer Set Value (M251) - Cycle Time Complete (M250).

Reasoning:

After the MOV instruction, M250 contains the counted down time from the Timer Set Value (M251). To get the time it took from the Output turning ON to OFF from 0, M251 is subtracted by M250.

Note: 

1. The Timer Set Value (M251) needs to be initialized to a value big enough to capture the cycle time of the task. The largest value accepted in the TMR Set Value is 6553.5 seconds (65535).
2. Auxiliary Relay (#76000) is not being used in this example, but could be useful in determining if the the event time took longer than what was recorded.