Detailed Explanation of Digital, Analog, and Pulse Signals

Digital signals, analog signals, and pulse signals are three very common types of signals used in automation systems.

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1. Digital Signals (Discrete / Switching Signals)

Digital signals (also called switching signals) have only two possible states:

• 0 or 1
• ON or OFF
• Presence or absence
  
  

In other words, there are only two results.

Typical input examples

• Push buttons — pressed / not pressed
• Proximity sensors — object detected / not detected
• Limit switches — triggered / not triggered
• Counting detection — pulse received / not received
  
  

Whenever a signal has only two states, we use digital input.

Typical output uses

• Motor start / stop
• Lamp ON / OFF
• Solenoid valve open / close
  
  

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2. Analog Signals (Continuous Signals)

Analog signals are values that continuously exist and continuously change.

Examples:

• Temperature
• Pressure
• Weight
• Liquid level
• Speed (variable control)
  
  

You cannot say “temperature exists or not”; it is always present and changing.
Therefore it is called an analog quantity.

How analog values enter a control system

Physical quantities must be converted into electrical signals:

Common standards:

• Current: 0–20 mA, 4–20 mA
• Voltage: 0–5 V, 0–10 V
  
  

Example

A temperature sensor range: 0–100°C → 0–10 V

TemperatureVoltage
0°C0 V
30°C3 V
100°C10 V

So when the controller receives 3 V, it calculates the temperature as 30°C.

Pressure, weight, and level work the same way.

Analog output control

Used when the target has adjustable levels:

• Valve opening: 0–100%
• Motor speed control
• Force control
• Heating power control
  
  

Example:
0–10 V controls valve opening
5 V → 50% open

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3. Pulse Signals (High-Speed Digital Signals)

Pulse signals are actually a special type of digital signal —
but continuous and regular.

Like a heartbeat: continuously ON and OFF in a regular pattern.

Typical uses

• Encoders
• High-speed counting
• Position measurement
• Stepper/servo positioning
  
  

If 10 pulses = 10 units of movement
Receiving 10 pulses means the object moved 10 units.

Speed control using pulses

Speed depends on pulse frequency:

• Send 10 pulses in 1 second → fast movement
• Send 10 pulses in 1 minute → slow movement
  
  

So controlling pulse interval = controlling speed

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SummarySignal TypeCharacteristicsTypical Use
DigitalOnly ON/OFFSwitches, sensors, start/stop
AnalogContinuous changing valueTemperature, pressure, speed
PulseRegular repeated digital signalCounting, position, encoder

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Understanding these three signal types is fundamental in automation because every PLC system processes inputs and outputs using one (or a combination) of them.