4–20mA Current Signal vs RS485 Differential Signal

4–20mA Current Signal vs RS485 Differential Signal: Principles, Differences and Industrial Applications

In industrial automation, process control and sensor systems, field devices need to transmit temperature, pressure, level, flow, water-quality or other measurement data to PLCs, DCS, RTUs and data-acquisition systems. 4–20mA and RS485 are two widely used field signal methods, but they serve different purposes.

4–20mA is mainly used to transmit a continuous analog process variable, while RS485 is a differential serial communication interface for digital data. Both are suitable for industrial environments and relatively long communication distances, but they differ significantly in data capacity, wiring, diagnostics and system integration.

1. What Is a 4–20mA Current Signal?

4–20mA is a standard analog current signal widely used in industrial process control. The lower end of the measuring range is normally represented by 4mA and the upper end by 20mA, with intermediate process values represented proportionally between them.

For example, consider a temperature transmitter with a measuring range of 0–100°C:

For a linear 0–100°C range, temperature can be calculated from the current using:

T = (I − 4) ÷ 16 × 100

Where T is temperature and I is current in mA. At 12mA, the corresponding temperature is 50°C.

2. Why Is 4–20mA Used Instead of 0–20mA?

The 4mA lower limit is commonly called a live zero. Even when the process variable is at the bottom of its measuring range, current still flows in the loop. This allows the control system to distinguish a valid zero measurement from certain wiring faults.

For example, 0% process value normally corresponds to 4mA, while a completely open circuit may result in current close to 0mA. Industrial transmitters can also use defined fault-current levels below 4mA or above 20mA, depending on the device and control-system configuration.

3. Main Advantages of 4–20mA

Good Noise Immunity

Current transmission performs well in electrically noisy industrial environments.

Suitable for Long Cable Runs

Long-distance analog transmission is possible when the loop voltage budget is sufficient.

Live-Zero Diagnostics

A 4mA lower limit helps distinguish normal minimum measurement from certain open-circuit conditions.

Two-Wire Operation

A two-wire transmitter can use the same pair of wires for power and 4–20mA signal transmission.

High PLC Compatibility

Many PLC, DCS and data-acquisition modules provide standard 4–20mA analog inputs.

4. Does Cable Resistance Affect a 4–20mA Loop?

Within its normal operating range, the transmitter regulates the loop current so that it corresponds to the measured process value. This makes 4–20mA less sensitive to cable resistance than direct voltage transmission.

Cable resistance still creates voltage drop, so it must be included in the system design. The power supply must provide enough voltage for the transmitter itself, cable resistance, PLC input resistance, isolators and any other series-connected devices.

This is commonly evaluated using a loop voltage budget:

Supply Voltage ≥ Transmitter Minimum Voltage + Cable Drop + Load Drop + Other Series Device Drops

5. How Is 4–20mA Converted to a Voltage Signal?

Some acquisition systems use a precision resistor to convert loop current into voltage. With an approximately 250Ω resistor:

This makes it possible to convert a 4–20mA signal into an approximately 1–5V input for analog acquisition circuits. Where the PLC or input module already supports current input directly, wiring should follow the module manufacturer's instructions.

6. Is 4–20mA Automatically Intrinsically Safe?

A 4–20mA signal is a signal standard and does not automatically make a system intrinsically safe.

Intrinsic-safety design limits voltage, current, power, inductance and capacitance across the entire circuit and requires certified field devices, barriers, isolators, power supplies and wiring practices appropriate to the hazardous-area classification.

7. What Is an RS485 Differential Signal?

RS485 is an industrial serial communication electrical interface standard. It uses a pair of differential signal lines to transmit digital data, and the receiver determines the logical state from the voltage difference between the two conductors.

A common two-wire RS485 network uses a twisted pair usually labeled A and B, although some equipment uses labels such as D+ and D−. A/B polarity naming can differ between manufacturers, so wiring should follow the terminal definition of the specific device.

RS485 defines the physical electrical layer. It does not define how temperature, pressure or other measurements are encoded. Industrial devices commonly run Modbus RTU or another higher-level protocol over RS485.

8. Why Does RS485 Have Good Noise Immunity?

RS485 uses differential signaling. External electrical noise that couples similarly onto both conductors appears largely as common-mode interference, while the receiver primarily evaluates the voltage difference between the two lines.

Twisted-pair cable further improves noise rejection. Long-distance or electrically noisy installations may also require suitable shielding, grounding, termination and surge protection.

9. How Far Can RS485 Communicate?

RS485 is suitable for long-distance serial communication, but maximum distance depends on baud rate, cable characteristics, network topology, termination, transceiver design and electromagnetic environment.

A distance of approximately 1200m is often quoted as a typical long-range value under favorable conditions and relatively low communication speed. As baud rate increases, the maximum reliable distance generally decreases.

Communication distance and baud rate should therefore be designed together rather than treating 1200m as a guaranteed value for every RS485 network.

10. Is RS485 Limited to 32 Devices?

Traditional RS485 systems use the concept of Unit Load to define bus loading. With classic 1 Unit Load transceivers, a network is commonly designed for up to 32 unit loads.

Modern transceivers can use 1/2, 1/4 or 1/8 Unit Load input designs, allowing 64, 128 or even 256 electrical nodes under suitable conditions.

The practical device count is also affected by protocol address range, polling cycle, cable length, topology and response time.

11. Is RS485 Always Half-Duplex?

RS485 can support both half-duplex and full-duplex structures.

Industrial RS485 Modbus RTU sensors commonly use two-wire half-duplex communication, which is why A/B two-wire communication is widely seen in field instrumentation.

12. What Is the Relationship Between RS485 and Modbus RTU?

RS485 and Modbus operate at different layers of the communication system.

RS485

Defines the electrical interface and how differential signals are transmitted over the cable.

Modbus RTU

Defines device addresses, function codes, register organization and data frames.

An “RS485 Modbus RTU sensor” therefore normally means:

Physical Interface: RS485
Communication Protocol: Modbus RTU

A PLC or data logger can read multiple parameters such as temperature, pressure, level, water quality, soil measurements and device status through Modbus registers.

13. Are Baud Rate, Data Bits, Parity and Stop Bits Part of RS485?

Baud rate, data bits, parity and stop bits are serial communication frame settings rather than fixed parameters defined by the RS485 electrical standard.

A typical Modbus RTU device may use settings such as:

Both ends of the communication link must use matching serial settings and the same higher-level protocol.

14. Does RS485 Define ASCII or Binary Data?

RS485 only defines the electrical transmission method and does not require ASCII or binary encoding.

For example, Modbus RTU uses a binary frame structure, while Modbus ASCII uses ASCII characters. Other equipment manufacturers can also use proprietary protocols over an RS485 interface.

15. Why Are Termination Resistors Used on RS485 Networks?

At long cable lengths or higher communication speeds, the RS485 cable behaves as a transmission line. Signal reflections at the ends of the cable can distort the waveform and cause communication errors.

Suitable termination resistors are therefore commonly placed at both ends of the bus according to the cable characteristic impedance. Many industrial RS485 networks use approximately 120Ω twisted-pair cable and matching termination where required.

Termination should be selected according to communication distance, baud rate, topology and transceiver requirements.

16. What Are the Main Differences Between 4–20mA and RS485?

17. When Should 4–20mA Be Selected?

4–20mA is well suited to systems where a stable continuous process variable is the main requirement, especially where an existing PLC or DCS already provides analog input modules.

Pressure transmitters

Level transmitters

Temperature transmitters

Flow transmitters

Process analyzers

Traditional PLC / DCS control systems

Where only one primary measured value is required and the system should remain simple and compatible with conventional industrial control architecture, 4–20mA is often a practical choice.

18. When Should RS485 Be Selected?

RS485 is suitable for projects that require multiple digital parameters, multi-device networking or direct communication with PLCs, RTUs, data loggers and IoT gateways.

Multi-parameter water-quality sensors

Multi-parameter soil sensors

Weather sensors

Digital level and flow instruments

Distributed environmental monitoring

PLC / RTU / IoT gateway integration

RS485 Modbus RTU also allows multiple devices to share one bus, which can reduce communication cabling in distributed monitoring systems.

19. Can 4–20mA and RS485 Be Used at the Same Time?

Yes. Some industrial instruments provide both 4–20mA and RS485 outputs, allowing the same measurement to be integrated into a conventional analog control system while additional digital parameters are accessed through communication.

For example, 4–20mA can be used as the primary real-time PLC control signal, while RS485 Modbus RTU is used to read measurement values, compensation data, device status or configuration parameters. Simultaneous use depends on the interface design of the specific instrument.

FAQ

Q1: Is RS485 a current signal or a voltage signal?

A1: RS485 is a differential voltage communication interface. The receiver determines the logical state from the voltage difference between the two signal conductors.

Q2: Does cable resistance affect a 4–20mA loop?

A2: Cable resistance creates voltage drop. When the transmitter remains within its operating voltage range and the loop voltage budget is sufficient, the loop current can still represent the correct process value.

Q3: Can RS485 always communicate up to 1200m?

A3: Actual distance depends on baud rate, cable, termination, topology and electromagnetic environment. Lower communication speeds and good wiring conditions are more favorable for long-distance communication.

Q4: Is RS485 limited to 32 devices?

A4: Thirty-two Unit Loads is the classic RS485 loading reference. Modern lower-Unit-Load transceivers can support more electrical nodes, while the practical count also depends on the communication protocol and network design.

Q5: Is RS485 always half-duplex?

A5: No. RS485 can use two-wire half-duplex or four-wire full-duplex structures. Industrial Modbus RTU sensors commonly use two-wire half-duplex communication.

Q6: Are RS485 and Modbus the same protocol?

A6: No. RS485 defines the physical electrical interface, while Modbus RTU defines device addresses, registers, function codes and data frames.

Q7: Is 4–20mA automatically intrinsically safe?

A7: No. Intrinsic safety depends on the complete loop energy limits, device certification, barriers and installation design rather than the 4–20mA signal alone.

Q8: Which is better for industrial sensors, 4–20mA or RS485?

A8: 4–20mA is well suited to single process variables and conventional PLC analog control. RS485 Modbus RTU is generally more suitable for multi-parameter sensors, device networking and digital data acquisition.

Conclusion

4–20mA and RS485 are both established industrial field communication methods, but they solve different problems. 4–20mA uses analog current to represent a continuous process variable and offers simple architecture, good noise immunity, long-distance capability and broad compatibility with conventional PLC and DCS systems.

RS485 uses differential voltage signaling for digital communication. It can connect multiple devices on one bus and, together with protocols such as Modbus RTU, can carry multiple measured values, status information and configuration data. Communication distance and node count should be designed according to baud rate, cable, transceiver loading and network topology.

METRAVON industrial measurement products can be configured with 4–20mA, RS485 Modbus RTU and other industrial interfaces according to project requirements, with integration options for PLCs, RTUs, DCS, data loggers and IoT gateways. The appropriate signal method should be selected according to parameter count, communication distance, existing control architecture, wiring conditions and future expansion requirements.