Terminology - Siemens Simatic S7 Series Manual

Sensors
Actuators.
Discrete Inputs and Outputs. Discrete inputs and outputs,.also.referred.to.as.digital

Terminology

Developing.an.understanding.of.PLCs.requires.learning.
some.basic.terminology..This.section.provides.an.overview.of.
commonly.used.PLC.terms,.beginning.with.the.terms.sensor.
and.actuator.
Sensors.are.devices.that.convert.a.physical.condition.into.an.
electrical.signal.for.use.by.a.controller,.such.as.a.PLC..Sensors.
are.connected.to.the.input.of.a.PLC..A.pushbutton.is.one.
example.of.a.sensor.that.is.often.connected.to.a.PLC.input..An.
electrical.signal.indicating.the.condition.(open.or.closed).of.the.
pushbutton.contacts.is.sent.from.the.pushbutton.to.the.PLC.
Actuators.are.devices.that.convert.an.electrical.signal.from.a.
controller,.such.as.a.PLC,.into.a.physical.condition..Actuators.are.
connected.to.the.PLC.output..A.motor.starter.is.one.example.of.
an.actuator.that.is.often.connected.to.a.PLC.output..Depending.
on.the.status.of.the.PLC.output,.the.motor.starter.either.
provides.power.to.the.motor.or.prevents.power.from.flowing.to.
the.motor.
Discrete Input
Sensor
Input
Point
Pushbutton
inputs.and.outputs,.are.either.on.or.off..Pushbuttons,.toggle.
switches,.limit.switches,.proximity.switches,.and.relay.contacts.
are.examples.of.devices.often.connected.to.PLC.discrete.
inputs..Solenoids,.relay.and.contactor.coils,.and.indicator.lamps.
are.examples.of.devices.often.connected.to.PLC.discrete.
outputs..
In.the.on.condition,.a.discrete.input.or.output.is.represented.
internal.to.the.PLC.as.a.logic...In.the.off.condition,.a.discrete.
input.or.output.is.represented.as.a.logic.0.
Discrete Output
PLC
Central
Processing Output
Point
Unit
(CPU)
2
Actuator
Motor
Motor
Starter