Siemens Simatic S7 Series Manual page 35

Basic Ladder Logic Symbols
Contacts
Coils.
STEP.7-Micro/WIN.has.three.editors.for.program.development,.
one.for.each.of.the.types.of.programming.available,.ladder.logic.
(LAD),.statement.list.(STL),.and.function.block.diagram.(FBD)..
The.STL.editor.is.often.preferred.by.experienced.programmers.
because.of.the.similarity.of.STL.programs.to.assembly.
language.computer.programs..However,.the.STL.editor.can.only.
be.used.with.the.SIMATIC.instruction.set..Both.the.LAD.and.
FBD.editors.can.be.used.with.either.instruction.set..Throughout.
this.course,.although.other.instruction.types.will.occasionally.be.
shown,.the.emphasis.will.be.on.SIMATIC.LAD.instructions.
PLC.ladder.logic.consists.of.a.commonly.used.set.of.symbols.
that.represent.instructions..Understanding.these.basic.symbols.
is.essential.to.understanding.PLC.operation.
One.of.the.most.confusing.aspects.of.PLC.programming.for.
first-time.users.is.the.relationship.between.the.device.that.
controls.a.status.bit.and.the.programming.function.that.uses.
a.status.bit..Two.of.the.most.common.programming.functions.
are.the.normally open (NO) contact.and.the.normally
closed (NC) contact..Symbolically,.power.flows.through.these.
contacts.when.they.are.closed..The.normally.open.contact.(NO).
is.closed.when.the.input.or.output.status.bit.controlling.the.
contact.is...The.normally.closed.contact.(NC).is.closed.when.
the.input.or.output.status.bit.controlling.the.contact.is.0.
Coils.represent.relays.that.are.energized.when.power.flows.
to.them..When.a.coil.is.energized,.it.causes.a.corresponding.
output.to.turn.on.by.changing.the.state.of.the.status.bit.
controlling.that.output.to...That.same.output.status.bit.may.be.
used.to.control.normally.open.and.normally.closed.contacts.
elsewhere.in.the.program.
34