When you select a pressure, level, flow or temperature sensor, the output signal matters as much as the sensing element. The choice between 4-20 mA vs RS485 Modbus affects wiring, cost, diagnostics and how easily the sensor fits into your control system. Both are proven industrial standards, but they solve different problems.
This article explains how each output works and how to choose between them.
How the 4-20 mA Current Loop Works
A 4-20 mA sensor represents its measurement as a current. The lowest value of the measuring range corresponds to 4 mA and the highest to 20 mA. For example, a 0-10 bar pressure transmitter outputs 4 mA at 0 bar, 12 mA at 5 bar and 20 mA at 10 bar.
Why current instead of voltage?
Current stays the same everywhere in a series loop. Wire resistance does not change the signal, as long as the power supply has enough voltage to drive the loop. This makes 4-20 mA very robust over long cable runs and in electrically noisy plants.
The live zero
Because the scale starts at 4 mA rather than 0 mA, the receiver can tell the difference between a true zero reading and a fault. A current of 0 mA means a broken wire or a dead sensor. Many devices also signal faults by driving the current outside the normal range, for example below 3.8 mA or above 20.5 mA, following the NAMUR NE 43 recommendation.
2-wire, 3-wire and 4-wire
- 2-wire (loop-powered): the same two wires carry power and signal. This is the most common and economical design.
- 3-wire: separate supply, with a shared return for power and signal.
- 4-wire: fully separate power and signal pairs, used for sensors that need more power.
Many 4-20 mA devices also support HART, which superimposes digital data on the analog signal for configuration and diagnostics.
How RS485 Modbus Works
RS485 is a physical layer standard. It uses a balanced differential pair of wires, which rejects common-mode noise and supports long cable runs, up to around 1200 m at lower data rates. Modbus RTU is the protocol most often used on top of it.
Multi-drop network
The main advantage of RS485 is that many devices can share one cable. A single master, such as a PLC or gateway, polls each sensor by its address. A standard RS485 segment supports up to 32 unit loads, and more with low-load transceivers or repeaters. Modbus addresses run from 1 to 247.
More than one value
A Modbus sensor can report several measurements over the same connection. A temperature and humidity sensor, for example, can provide temperature, relative humidity, dew point and status bits, all stored in registers.
Wiring rules
RS485 works reliably when you respect a few rules:
- Use twisted-pair shielded cable.
- Wire devices in a daisy chain, not a star.
- Fit a termination resistor (typically 120 ohms) at both ends of the bus.
- Make sure all devices share the same baud rate, parity and stop-bit settings.
- Give every device a unique address.
Comparison Table
| Criterion | 4-20 mA | RS485 Modbus RTU |
|---|---|---|
| Signal type | Analog current | Digital, differential |
| Values per device | One (plus HART if supported) | Many registers |
| Wiring | One pair per sensor | One bus for many sensors |
| Typical distance | Long, limited by loop voltage | Up to about 1200 m |
| Noise immunity | Very good | Very good |
| Fault detection | Live zero, out-of-range current | Communication errors, status registers |
| Configuration | Usually on device or via HART | Remote, via registers |
| Setup effort | Very simple | Needs addressing and settings |
| Update speed | Continuous | Depends on polling cycle |
| Best for | Critical control loops, simple retrofits | Multi-sensor monitoring, data logging, IoT |
When to Choose 4-20 mA
Choose 4-20 mA when:
- The signal feeds a fast control loop, such as a PID controller.
- Your PLC already has free analog inputs.
- You need a simple, universally understood signal that any technician can check with a multimeter.
- The application involves safety functions or hazardous areas where analog loops are well established.
- You only need one measurement per sensor.
When to Choose RS485 Modbus
Choose RS485 Modbus when:
- You have many sensors spread across a site and want to reduce cabling.
- You need several values from each device, or diagnostic data.
- You want to configure ranges and alarms remotely.
- Data goes to a SCADA system, data logger or IoT gateway rather than a control loop.
- Your controller has few analog inputs but a free serial port.
Can You Combine Both?
Yes, and many installations do. A common approach is to use 4-20 mA for critical control signals and RS485 Modbus for monitoring and energy data. Some sensors offer both outputs at once. Signal converters can also translate 4-20 mA into Modbus registers, which is useful when you want to bring older analog devices into a digital monitoring system.
Troubleshooting Tips
For 4-20 mA loops: – Check the loop supply voltage under full load. – Measure the current in series with a meter. – A reading of 0 mA points to a wiring break or power fault.
For RS485 Modbus: – Confirm termination and biasing resistors. – Check that A and B lines are not swapped. – Verify address, baud rate and parity on every device. – Look for star wiring or long stubs, which cause reflections.
Making the Decision: 4-20 mA vs RS485 Modbus
The 4-20 mA vs RS485 Modbus decision comes down to what you need from the data. For a single, robust value feeding a control loop, 4-20 mA remains hard to beat. For many sensors, multiple values and remote configuration, RS485 Modbus is more efficient.
Most sensor categories in our catalogue offer both output options. Browse the product pages to compare versions, or contact our team for help matching a sensor output to your PLC or gateway.