Siemens Simatic S7 Series Manual page 40

Forcing.
When.the.toggle.switch.closes,.input.point.I2..turns.on.and.
I2..status.bit.changes.to.a.logic...This.causes.normally.open.
contact.I2..to.close.and.turn.on.Q3..coil..Note.that.a.closed.
contact.and.a.coil.that.is.on.are.shown.highlighted.in.the.
program..When.Q3..coil.turns.on,.the.Q3..status.bit.goes.to.a.
logic..and.output.point.Q3..turns.on..This.causes.the.lamp.to.
light.
Toggle Switch
Input
Point
I2.1
ON
Forcing.is.another.useful.tool.in.the.startup.and.maintenance.
of.a.PLC.system..Forcing.overrides.one.or.more.input.or.output.
status.bits,.causing.them.to.stay.in.either.a.logic.0.or.logic..
status..
For.example,.in.the.following.illustration,.the.toggle.switch.is.
open..Under.normal.circumstances,.the.toggle.switch.would.
have.to.be.closed.to.turn.on.the.lamp..However,.if.the.I2..
status.bit.is.forced.to.a.logic.,.the.lamp.will.turn.on,.as.long.as.
the.program.is.functioning.correctly.and.there.are.no.hardware.
or.wiring.problems..Similarly,.the.Q3..status.bit.could.be.forced.
to.a.logic..to.turn.on.the.lamp.
Toggle Switch
Input
Point
I2.1
OFF
Toggle Switch
Input
Point
I2.1
OFF
Forcing.is.useful.not.only.to.test.and.debug.programs.and.
hardware.during.startup,.but.also.to.troubleshoot.systems.with.
problems.
CPU Program
I2.1
Input
Status Bit
I2.1
Logic 1
CPU Program
I2.1
Input
Status Bit
I2.1
Logic 0
CPU Program
I2.1
Input
Status Bit
I2.1
Logic 1
39
Q3.1
Output
Status Bit
Q3.1
Logic 1
Q3.1
Output
Status Bit
Q3.1
Logic 0
Q3.1
Output
Status Bit
Q3.1
Logic 1
Output
Point
Q3.1 Lamp
ON
Output
Point
Q3.1 Lamp
OFF
Output
Point
Q3.1 Lamp
ON