7.5 Alternate ON-OFF output
Example makes a certain number of impulses of desired duration at PLC controller IR010.00 output. Number of impulses is given in instruction of the counter (here it is a constant #0010 or ten impulses) impulse duration in two timer instructions. First timer defines duration of ON status, and second one duration of OFF status of IR010.00 output bit. In the example these two durations are the same, but through assigning them different parameters they can differ so that duration of ON status can be different from duration of OFF status.
Program starts executing a condition at IR000.00 bit. Since a normally closed contact which refers to counter flag (that isn't set ) is linked with this IR000.00 bit in "I" circuit, this status of IR200.00 bit will change to ON. Bit IR200.00 keeps its status through self-maintenance until counter flag is not set and a condition interrupted.
When an IR200.00 bit is set, timers TIM001 and TIM002 start counting a set interval number at 0.1 s ( in the example, this number is 10 for the first timer, or 20 for the second timer, and this sets the period of one or two seconds). With both timers, a normally closed contact which refers to TIM002 timer flag is connected with IR200.00 bit. When this flag is set which happens every two seconds, both timers are reset. Timer TIM002 resets timer TIM001 and itself, and this starts a new cycle.
At the start of a program, IR010.00 output bit changes status to ON and stays in this status until TIM001 flag changes status to ON (after one second). By changing TIM001 flag status to ON, condition is broken (because it is represented as normally closed contact) and IR010.00 bit changes status to OFF.
IR010.00 output status changes to ON again when time has run out on TIM002 timer. This resets TIM001 timer and its flag which in turn executes a condition for status change of the IR010.00 output. Cycle is thus repeated until a counter numbers 10 changes of TIM001 flag status. With the change of status of CNT000 counter flag, a condition for an assisting bit IR200.00 is broken, and program stops working.
Ladder Diagram:
