In industrial automation systems, sensors and instruments need to communicate measurement information to PLCs, DCS systems, and control equipment.
Among all industrial signals, analog signals and digital signals are the two most commonly used signal types.
For beginners in automation and instrumentation, it is sometimes confusing:
- What is the difference between analog and digital signals?
- Why do most industrial transmitters use 4-20mA signals?
- What are DI, DO, AI, and AO in PLC systems?
- Which instruments use analog signals and which use digital signals?
Today, we will explain the key differences between analog signals and digital signals, as well as their practical applications in industrial measurement and control systems.
1. What Is a Digital Signal (Switch Signal)?
A digital signal, also called a switch signal, only has two possible states:
- ON / OFF
- High / Low
- 1 / 0
In automation systems, digital signals are usually used for simple status detection or equipment control.
For example:
- Motor running / stopped
- Valve open / closed
- Pump start / stop
- High-level alarm / low-level alarm
A PLC does not know the exact value of a digital signal. It only recognizes whether the signal exists or not.
Example:
A pressure switch installed on a pipeline:
Pressure < 5 bar → OFF
Pressure ≥ 5 bar → ONThe PLC only receives:
0 or 1It does not know whether the pressure is 6 bar, 8 bar, or 10 bar.
2. Common Digital Signal Types in PLC Systems
In PLC terminology:
Digital Input (DI)
DI means Digital Input.
It is a signal received by the PLC from field devices.
Typical DI devices include:
- Pressure switches
- Level switches
- Limit switches
- Proximity sensors
- Flow switches
- Relay contacts
Example:
A level switch installed on a storage tank:
Liquid level high → Switch ON
Liquid level normal → Switch OFFThe PLC receives the tank level status through a DI channel.
Digital Output (DO)
DO means Digital Output.
It is a control signal sent from the PLC to external equipment.
Typical DO applications:
- Motor start/stop
- Solenoid valve control
- Alarm activation
- Relay control
Example:
PLC sends a DO signal:
DO = ON
↓
Motor starter energized
↓
Motor starts3. What Is an Analog Signal?
Unlike digital signals, analog signals represent continuously changing values.
Industrial measurement parameters such as:
- Pressure
- Flow
- Temperature
- Level
- Density
are usually transmitted as analog signals.
Common industrial analog signals include:
4-20mA Current Signal
0-10V Voltage Signal
1-5V Voltage Signal
Among them, 4-20mA is the most widely used signal in industrial instrumentation.
4. Why Is 4-20mA Widely Used in Industry?
Most industrial transmitters use 4-20mA because it has several advantages:
1. Strong Anti-Interference Capability
Current signals are less affected by electrical noise compared with voltage signals.
This makes them suitable for long-distance transmission in industrial plants.
2. Long Transmission Distance
4-20mA signals can normally be transmitted hundreds of meters without significant signal loss.
They are widely used in:
- Chemical plants
- Oil & gas facilities
- Power plants
- Water treatment plants
3. Easy Fault Detection
The range starts from 4mA instead of 0mA.
Example:
Pressure transmitter:
Measurement range:
0-10 barSignal output:
4mA = 0 bar
12mA = 5 bar
20mA = 10 barIf the PLC receives:
0mAit can identify a possible fault:
- Cable disconnected
- Power failure
- Transmitter failure
5. Analog Input (AI) and Analog Output (AO)
In PLC systems:
Analog Input (AI)
AI receives analog signals from field instruments.
Typical AI devices:
- Pressure transmitters
- Flow meters
- Radar level transmitters
- Temperature transmitters
Example:
A pressure transmitter:
Pressure transmitter
↓
4-20mA signal
↓
PLC AI module
↓
Pressure value displayed on HMIAnalog Output (AO)
AO sends analog control signals from PLCs to field devices.
Typical AO applications:
- Control valves
- Variable frequency drives (VFD)
- Motor speed control
Example:
PLC controls a control valve:
PLC
↓
4-20mA output
↓
Control valve actuator
↓
Valve opening adjustment
6. Examples of Analog Signals in Industrial Instruments
Flow Meter
A flow meter can output:
- 4-20mA
- Pulse output
- RS485 Modbus communication
For example:
Electromagnetic flow meter:
0 m³/h
↓
4mA
100 m³/h
↓
20mAThe PLC or DCS system can continuously monitor the flow rate.
Pressure Transmitter
Example:
Range:
0-16 barOutput:
4mA = 0 bar
20mA = 16 bar
Radar Level Transmitter
Example:
Tank height:
0-10 metersOutput:
4mA = Empty
20mA = Full
7. Can Digital Signals and Analog Signals Be Converted?
Yes.
Signal conversion can be achieved through different modules.
A/D Conversion
A/D means:
Analog to Digital
It converts analog signals into digital data.
Example:
Pressure transmitter
4-20mA
↓
PLC AI module
↓
Digital value inside PLCD/A Conversion
D/A means:
Digital to Analog
It converts digital values into analog output signals.
Example:
PLC calculation
↓
AO module
↓
4-20mA output
↓
Control valve adjustment8. Comparison Between Analog and Digital Signals
| Item | Digital Signal | Analog Signal |
|---|---|---|
| Signal Type | ON/OFF | Continuous value |
| PLC Interface | DI / DO | AI / AO |
| Typical Signal | 0/1 | 4-20mA, 0-10V |
| Application | Status control | Measurement |
| Examples | Switches, Relays | Flow meters, Transmitters |
| Information | Two states only | Actual measured value |
9. Summary
Digital signals and analog signals are the foundation of modern industrial automation systems.
Digital signals are mainly used for:
- Equipment status
- Start/stop control
- Alarm signals
Analog signals are mainly used for:
- Continuous measurement
- Process monitoring
- Automatic control
In industrial instrumentation, signals such as 4-20mA, AI, AO, DI, and DO are essential communication methods between field instruments and control systems.
Understanding these signal types helps engineers correctly select, install, and troubleshoot industrial equipment such as:
- Flow meters
- Pressure transmitters
- Level transmitters
- Temperature sensors
- Control valves