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The output A2.4 should be activated for an instant each second.
first consider path 2 of the ladder diagram and the second group of AWL statements. The first time the program is run finds the timer stopped; therefore its NC contact on the ladder diagram and the query on its low state in the AWL program gives the result as true. In both cases the output of the Off-delay timer is put into high state. For this reason the above queries will not be verified during the next cycle. This means that the input to the timer has been set to zero and the time begins to advance. The output remains high throughout this period of time. When the time has elapsed, it is set to zero. This means that the conditions for querying the input to the timer are again fulfilled and the whole process repeats itself. The first line of the above diagram describes the development of the output from the timer which happens to be opposite to that required by the trace. All that remains is to invert this signal and assign it to output A2.4. This task is performed by the first path of the ladder diagram and the first group of statements in the AWL program.
The only thing remaining is to clarify that the path assigning the value to A2.4 should precede the one that activates the timer. Otherwise the time would be restarted before A2.4 can be programmed to 1. A2.4 is constantly maintained in low state as the status of T3, from the point of view of the development of the program, would always appear high. The above is also valid for the AWL program where the group of statements for the assignation of A2.4 should precede the initialisation of T3 for the same reasons. These considerations should be kept in mind whenever using a pulse train in any of the following examples.
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