Introduction
For anyone working in industrial instrumentation and automation, 4-20mA signals are very familiar.
Pressure transmitters, temperature sensors, level transmitters, and flow meters have been using this method for decades. Connect two wires, send the current signal back to the analog input module, and the control system can display the measurement value.
When a signal is abnormal, engineers can simply measure the loop current and check the wiring step by step. This simplicity and reliability are the main reasons why 4-20mA is still widely used today.
However, industrial instruments are becoming smarter.
Modern transmitters, flow meters, valve positioners, and analyzers are no longer just measurement devices. They can provide:
- Self-diagnostics
- Device status information
- Multiple process variables
- Alarm information
- Configuration parameters
- Firmware information
The challenge is that a traditional 4-20mA loop mainly transfers only one primary process variable.
Many valuable diagnostic data inside the instrument cannot be fully utilized.
This is where Ethernet-APL comes into play.
1. What Is Ethernet-APL?
Ethernet-APL (Ethernet Advanced Physical Layer) is an Ethernet-based physical layer technology designed specifically for process automation applications.
It is based on 10BASE-T1L Single Pair Ethernet (SPE) technology.
Key features include:
- Communication speed up to 10 Mbit/s
- Full-duplex Ethernet communication
- Data transmission and power supply through the same pair of wires
- Long-distance communication suitable for industrial plants
- Designed for hazardous area applications
It is important to understand that:
Ethernet-APL is not a communication protocol like PROFINET, EtherNet/IP, or HART-IP.
Ethernet-APL provides the physical connection layer that allows field instruments to connect directly to Ethernet networks.
It does not define how instrument data is structured or how device parameters are managed.
2. Why Is 4-20mA Still Popular?
The advantages of 4-20mA are clear.
A transmitter converts the measured value into a current signal:
Example:
A pressure transmitter with a measuring range of 0–10 bar:
- 4 mA = 0 bar
- 20 mA = 10 bar
The relationship is simple and easy to understand.
Advantages of 4-20mA:
✔ Simple wiring
✔ High reliability
✔ Easy troubleshooting
✔ Mature technology
✔ Low maintenance cost
For a single measurement point, 4-20mA remains an excellent solution.
For example:
- One pressure measurement point
- One temperature measurement point
- One level measurement point
If the control system only needs one process value, there is usually no reason to replace it.
3. The Limitation of 4-20mA
The limitation is that a 4-20mA signal mainly carries one measurement value.
However, modern smart instruments contain much more information.
For example, a smart flow meter may also know:
- Sensor condition
- Empty pipe detection
- Electronics status
- Totalized flow
- Medium temperature
- Device alarms
- Calibration information
- Operating parameters
Traditional 4-20mA communication cannot transmit all this information.
HART communication can add digital communication capability, but its data transmission speed is limited and it is mainly used for configuration and maintenance.
4. Ethernet-APL vs 4-20mA Comparison
| Item | 4-20mA | Ethernet-APL |
|---|---|---|
| Technology | Analog current signal | Industrial Ethernet physical layer |
| Main purpose | Transfer process value | Digital instrument communication |
| Data capacity | Mainly one variable | Multiple variables, parameters, diagnostics |
| Communication | Mainly one-way output | Bidirectional communication |
| Diagnostics | Limited, often requires HART/tools | Integrated device diagnostics |
| Wiring | Analog input module | APL switch/network infrastructure |
| Device management | Tag and loop based | Device identity, parameters, status |
| Best application | Simple measurement points | Smart instruments and digital plants |
The difference can be summarized simply:
4-20mA sends a measurement value.
Ethernet-APL connects the instrument as a digital device.
5. What Are the Advantages of Ethernet-APL?
The biggest advantage of Ethernet-APL is not improving measurement accuracy.
It is about making instrument information available.
For example, a smart flow meter can send:
- Flow rate
- Temperature
- Density
- Totalized value
- Sensor health
- Diagnostic messages
- Device status
directly to:
- DCS
- PLC systems
- Asset management platforms
- Digital monitoring systems
Better Troubleshooting
With traditional analog signals, the control system may only show:
- Signal too high
- Signal too low
- Communication failure
Engineers still need to determine whether the problem comes from:
- Cable failure
- Input module
- Instrument hardware
- Process conditions
Ethernet-APL can provide more detailed diagnostic information, such as:
- Sensor problems
- Electronics faults
- Parameter changes
- Communication status
This helps reduce troubleshooting time.
Remote Configuration and Maintenance
Ethernet-APL allows engineers to access instrument information remotely.
Without physically visiting the device, engineers can check:
- Measuring range
- Engineering units
- Damping settings
- Alarm settings
- Device identification
- Firmware version
For plants with thousands of instruments, this capability can significantly improve maintenance efficiency.
6. When Should You Continue Using 4-20mA?
4-20mA is still the right choice for many applications.
Typical examples:
- Simple pressure measurement
- Basic temperature monitoring
- Standard level measurement
- Existing plant upgrades
- Safety-related loops
The advantages remain:
- Simple design
- Easy maintenance
- Low investment cost
Replacing every analog loop with Ethernet communication is unnecessary.
7. When Should You Consider Ethernet-APL?
Ethernet-APL becomes attractive when:
- Many smart instruments are installed
- Device diagnostics are important
- Large amounts of instrument data are required
- Digital plant management is planned
Typical applications include:
- Multivariable flow meters
- Smart valve positioners
- Online analyzers
- Advanced pressure transmitters
- Digitalized process plants
For new industrial projects, Ethernet-APL can be considered during the system design stage.
8. Things to Consider Before Using Ethernet-APL
Ethernet-APL is not simply replacing a cable with an Ethernet connector.
A complete system requires:
- APL-compatible field instruments
- APL switches
- Network infrastructure
- Compatible control systems
- Asset management systems
For hazardous areas, engineers must also consider:
- Explosion protection requirements
- Cable distance
- Power limitations
- Equipment certification
The maintenance method will also change.
Traditional troubleshooting:
Measure current → Check wiring → Check module
Ethernet-APL troubleshooting:
Check network → Check device status → Analyze diagnostics
It is not only a wiring change, but also a change in maintenance philosophy.
9. Conclusion: Ethernet-APL Will Not Simply Replace 4-20mA
4-20mA is still an excellent solution for simple measurement applications.
Ethernet-APL provides a new approach for intelligent instrumentation by allowing measurement values, diagnostics, parameters, and device information to be integrated into digital systems.
The future is not about completely replacing 4-20mA.
Instead:
4-20mA is ideal for reliable signal transmission.
Ethernet-APL is designed for intelligent instruments, digitalization, and advanced process automation.
Both technologies will continue to exist together depending on application requirements.