One of the functions provided by some PLC manufacturers is the ability to program an internal relay so that its contacts are activated for just one cycle, that is, one scan through the ladder program. Hence when operated, the internal relay provides a fixed duration pulse atits contacts. This function is often termed one-shot. Though some PLCs have such a function aspart of their programs, such a function can also easily be developed with just two rungs of a ladder program. Figure shows such a pair of rungs.
For Figure when the trigger input occurs, it gives a trigger output in rung 1. In rung 2 it gives a cycle control output on an internal relay. Because rung 2 occurs after rung 1, the effect of the cycle control is not felt until the next cycle of the PLC program, when it opens the cycle control contacts in rung 1 and stops the trigger output. The trigger output then remains off, despite there being a trigger input. The trigger output can only occur again when the trigger output is switched off and then switched on again.
Figures show the built-in facilities with Allen-Bradley and Mitsubishi PLCs. With the Mitsubishi PLC (Figure), the output internal relay—say, M100—is activated when the trigger input—say, X400—contacts close. Under normal circumstances, M100 would remain on for as long as the X400 contacts were closed. However, if M100 has been programmed for pulse operation, M100 only remains on for a fixed period of time—one program cycle. It then goes off, regardless of X400 being on. The programming instructions that would be used are LD X400, PLS M100. The preceding represents pulse operation when the input goes from off to on, that is, is positive-going. If, in Figure the trigger input is made normally closed rather than normally open, the pulse occurs when the input goes from on to off—in other words, is negative-going.
The IEC 1131-3 gives standards for the symbols for positive transition-sensing and negative transition-sensing coils. With the positive transition-sensing coil, if the power flow to it changes from off to on, the output is set on for one ladder rung evaluation. With the negative transition-sensing coil, if the power to it changes from off to on, the output is set on for one ladder rung evaluation.
Thus, for the ladder rung of Figure with the input off there is no output. When the input switches on, there is an output from the coil. However, the next and successive cycles of the program do not give outputs from the coil even though the switch remains on. The coil only gives an output the first time the switch is on.
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