Siemens Simatic S7 Series Manual page 16

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PLC Scan.
In.the.following.example,.the.program.segments.perform.the.
same.function..
Statement List (STL)
Network 1
LD I0.0
A I0.1
= Q0.0
Network 2
LD I0.4
O I0.5
= Q0.1
In.addition.to.LAD,.STL,.and.FBD,.multiple.other.types.of.
programming.languages.are.used.for.PLCs..Each.type.of.
programming.has.its.advantages.and.disadvantages..Factors.
such.as.application.complexity,.types.of.programming.available.
for.a.specific.PLC.model,.and.user.standards.and.preferences.
determine.which.type.of.programming.is.used.for.an.
application.
The.PLC.program.is.executed.as.part.of.a.repetitive.process.
referred.to.as.a.scan..A.PLC.scan.starts.with.the.CPU.
reading.the.status.of.inputs..Next,.the.application.program.is.
executed..Then,.the.CPU.performs.internal.diagnostics.and.
communication.tasks..Finally,.the.CPU.updates.the.status.of.
outputs..This.process.repeats.as.long.as.the.CPU.in.the.run.
mode..The.time.required.to.complete.a.scan.depends.on.the.
size.of.the.program,.the.number.of.I/Os,.and.the.amount.of.
communication.required.
Function Block Diagram (FBD)
Network 1
I0.0
AND
I0.1
Network 2
I0.4
OR
I0.5
PLC Scan
5
Ladder Logic (LAD)
Network 1
I0.0
Q0.0
Network 2
I0.4
Q0.1
I0.5
I0.1 Q0.0
Q0.0