Siemens Simatic S7 Series Manual page 64

Interrupts.
Pulse Training Output (PTO).
Assume,.for.example,.that.the.encoder.generates.600.pulses.
per.revolution,.and.it.takes.000.motor.revolutions.to.move.
the.platform.from.one.position.to.another,.moving.the.platform.
from.position..to.position.6.(5.positions).takes.5000.motor.
revolutions.or.30,000.encoder.pulses..In.most.practical.
applications,.the.frequency.of.these.pulses.is.too.high.for.them.
to.be.counted.with.inputs.that.are.not.associated.with.a.high-
speed.counter..
S7-200.PLCs.incorporate.instructions.for.use.with.interrupts..
Interrupts.are.used.to.initiate.a.specific,.short.PLC.program.
segment,.called.an.interrupt.routine,.when.an.internal.or.
external.event.occurs..After.the.interrupt.routine.has.been.
executed,.control.is.returned.to.the.main.program.
Three.types.of.interrupts.are.supported.by.S7-200.PLCs,.
communication port interrupts,.I/O interrupts,.and.time-
based interrupts..Communication.port.interrupts.are.used.
to.control.a.communication.port.operated.in.Freeport.mode..
I/O.interrupts.are.used.to.respond.quickly.to.high-speed.I/O.
transitions,.such.as.those.associated.with.high-speed.counters.
or.pulse.train.outputs..Time-based.interrupts.allow.the.user.
program.to.execute.an.interrupt.routine.on.a.cyclic.basis.
Each.of.these.types.of.interrupts.has.an.associated.priority.
that.determines.which.interrupt.is.processed.first.in.the.event.
that.two.or.more.interrupts.are.requested.at.the.same.time..
Communication.port.interrupts.have.the.highest.priority.and.
time-based.interrupts.have.the.lowest.priority.
S7-200.PLCs.have.two.PTO/PWM generators.that.create.either.
a.high-speed.pulse.train.or.a.pulse.width.modulated.waveform..
One.generator.is.assigned.to.output.point.Q0.0.and.the.other.to.
output.point.Q0...When.a.generator.is.activated,.it.controls.its.
respective.output.
Pulse Train Output (PTO).is.used.to.provide.a.series.of.pulses.
to.an.output.device,.such.as.a.stepper.motor.driver..The.PTO.
provides.a.square.wave.output.for.a.specified.number.of.pulses.
and.a.specified.cycle.time..The.number.of.pulses.can.be.from..
to.4,294,967 ,295.pulses..The.Pulse.Train.Output.has.a.50%.duty.
cycle..This.means.the.pulse.is.off.for.the.same.amount.of.time.
that.it.is.on.
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