Siemens Simatic S7 Series Manual page 15

Ladder Logic Programming
Statement List and
Function Block Diagrams.
A.program.consists.of.instructions.that.accomplish.specific.
tasks..The.degree.of.complexity.of.a.PLC.program.depends.
upon.the.complexity.of.the.application,.the.number.and.type.of.
input.and.output.devices,.and.the.types.of.instructions.used.
Ladder logic (LAD).is.one.programming.language.used.with.
PLCs..Ladder.logic.incorporates.programming.functions.that.are.
graphically.displayed.to.resemble.symbols.used.in.hard-wired.
control.diagrams..
The.left.vertical.line.of.a.ladder.logic.diagram.represents.the.
power.or.energized.conductor..The.output.coil.instruction.
represents.the.neutral.or.return.path.of.the.circuit..The.right.
vertical.line,.which.represents.the.return.path.on.a.hard-wired.
control.line.diagram,.is.omitted..Ladder.logic.diagrams.are.read.
from.left-to-right.and.top-to-bottom..Rungs.are.sometimes.
referred.to.as.networks..A.network.may.have.several.control.
elements,.but.only.one.output.coil.
Power Conductor
Network 1
I0.0 I0.1
Network 2
I0.4
I0.5
While.ladder.logic.programs.are.still.common,.there.are.many.
other.ways.to.program.PLCs..Two.other.common.examples.are.
statement.list.and.function.block.diagrams.
Statement list (STL).instructions.include.an.operation.and.an.
operand..The.operation.to.be.performed.is.shown.on.the.left..
The.operand,.the.item.to.be.operated.on,.is.shown.on.the.right.
Function block diagrams (FBD).include.rectangular.functions.
with.inputs.shown.on.the.left.side.of.the.rectangle.and.outputs.
shown.on.the.right.side.
4
Normally Open Contact Instructions
Q0.0
Output Coil Instruction
Q0.0