Digital Inputs vs Digital Outputs — PLC Basics Every Engineer Must Know
In every PLC system, signals are divided into two main types 
Inputs → Information coming INTO PLC
Outputs → Commands going OUT from PLC
Digital Input (DI)
Signal received by PLC from field devices
Only two states:
ON / OFF
1 / 0
Examples of Digital Inputs
Push buttons
Limit switches
Proximity sensors
Motor feedback signals
PLC reads the status
Digital Output (DO)
Signal sent from PLC to field devices
Also ON/OFF type signals
Examples of Digital Outputs
Relay outputs
Solenoid valves
Contactors
Alarm lamps & buzzers
PLC controls the device
Simple Flow
Digital Input → PLC reads condition
PLC executes logic
Digital Output → PLC performs action
Real Example
Start Push Button → Digital Input
Motor Contactor → Digital Output
Operator presses button → PLC energizes motor
Important for beginners
Inputs = Sensors / switches
Outputs = Actuators / control devices
Simple understanding
DI = “What is happening?”
DO = “What should happen?”
PLC works by reading inputs and controlling outputs continuously
PLC Digital Signals Exercise
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