Siemens Simatic S7 Series Manual page 39

Testing a Program.
Contact and Coil Status
Once.a.program.has.been.written,.it.needs.to.be.tested.and.
debugged..One.way.this.can.be.done.is.to.simulate.the.field.
inputs.with.an.input.simulator,.such.as.the.one.made.for.the.
S7-200.PLC..
The.program.is.first.downloaded.from.the.programming.device.
to.the.CPU..The.selector.switch.is.placed.in.the.RUN.position..
The.simulator.switches.are.operated.and.the.resulting.indication.
is.observed.on.the.output.status.indicator.lamps.
SF/DIAG
After.a.program.has.been.loaded.and.is.running.in.the.PLC,.the.
actual.status.of.ladder.elements.can.be.monitored.using.STEP.7.
Micro/WIN.software..
For.example,.in.the.following.illustration,.the.toggle.switch.
controls.the.status.bit.for.I2...As.long.as.the.toggle.switch.is.
open,.the.I2..status.bit.is.a.logic.0..The.I2..status.bit.controls.
the.I2..normally.open.contact..Because.the.I2..status.bit.is.
a.logic.0,.the.normally.open.contact.function.is.open.and.no.
power.is.passed.to.the.Q3..coil.function..As.a.result,.the.Q3..
status.bit.remains.a.logic.0.and.output.point.Q3..is.off.
Toggle Switch
Input
Point
I2.1
OFF
CPU Program
I2.1
Input
Status Bit
I2.1
Logic 0
38
Input Simulator
Q3.1
Output
Status Bit
Q3.1
Logic 0
Output
Point
Q3.1 Lamp
OFF