Siemens Simatic S7 Series Manual page 43

PLC Motor Control.
The.motor.will.run.until.the.normally.closed.Stop.button.
is.pressed,.unless.the.overload.relay.(OL).contacts.open..
When.the.Stop.button.is.pressed,.the.path.for.current.flow.is.
interrupted,.opening.the.associated.M.and.Ma.contacts,.and.
the.motor.stops.
This.motor.control.application.can.also.be.accomplished.
with.a.PLC..In.the.following.example,.a.normally.open.Start.
pushbutton.is.wired.to.the.first.input.(I0.0),.a.normally.closed.
Stop.pushbutton.is.wired.to.the.second.input.(I0.),.and.
normally.closed.overload.relay.contacts.(part.of.the.motor.
starter).are.connected.to.the.third.input.(I0.2)..These.inputs.are.
used.to.control.normally.open.contacts.in.a.line.of.ladder.logic.
programmed.into.the.PLC..
Start (NO) Input
Points
Network 1
I0.0
Stop (NC)
I0.1
OL
I0.2
Initially,.I0..status.bit.is.a.logic..because.the.normally.closed.
(NC).Stop.Pushbutton.is.closed..I0.2.status.bit.is.a.logic..
because.the.normally.closed.(NC).overload.relay.(OL).contacts.
are.closed..I0.0.status.bit.is.a.logic.0,.however,.because.the.
normally.open.Start.pushbutton.has.not.been.pressed..
Normally.open.output.Q0.0.contact.is.also.programmed.on.
Network..as.a.sealing.contact..With.this.simple.network,.
energizing.output.coil.Q0.0.is.required.to.turn.on.the.motor.
42
SF/D IAG
CPU Program
I0.0 I0.1 I0.2
Q0.0
Q0.0
Output
Point
Q0.0
Motor
Starter
Motor