RS485 vs 4-20mA: Which Sensor Output Should You Choose?
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Almost every industrial sensor you can buy comes in one of two output flavours: a 4–20mA analog current loop, or a digital RS485 interface running Modbus RTU. Both have been around for decades, both work well — but they suit very different systems. This guide walks through how each one works, where each one wins, and how to pick the right version before you order.
How each output works
4–20mA (current loop). The sensor converts its measurement into an electrical current: 4 mA represents the bottom of the measuring range and 20 mA the top. Your PLC or controller measures the current and scales it back into engineering units. Because 4 mA is a "live zero," a reading of 0 mA immediately tells you a wire is broken or the sensor has lost power. Each sensor needs its own pair of wires and its own analog input channel, and it can only ever report one value.
RS485 with Modbus RTU. The sensor is a digital device with its own address on a two-wire differential bus. A master (PLC, data logger, or gateway) polls the sensor's registers and gets exact digital values back — no scaling errors, no analog drift. One cable run can carry many sensors, and each sensor can report several parameters at once, plus configuration and diagnostic information.
Side-by-side comparison
| 4–20mA | RS485 / Modbus RTU | |
|---|---|---|
| Signal type | Analog current | Digital data |
| Values per device | One | Many (multi-parameter sensors report all values over one connection) |
| Wiring | One pair per sensor | One shared bus for up to 32+ devices |
| Typical max distance | 1,000 m or more, depending on loop supply and load | Up to 1,200 m at low baud rates |
| Fault detection | Wire break / power loss (0 mA) | Full diagnostics, error codes, device ID |
| Configuration | Scaling set at the receiver | Address, baud rate and registers set in software |
| Receiver needs | Analog input channel per sensor | One serial port + Modbus master software |
| Best at | Simple, rugged single-point measurement | Multi-sensor, multi-parameter systems |
Choose 4–20mA if…
- Your PLC or DCS already has spare analog input channels
- You are measuring one value at one point
- The run is long and electrically noisy — current loops are extremely resistant to interference
- You want the simplest possible setup: wire it, scale it, done
Choose RS485 Modbus if…
- You have several sensors to install — one bus cable replaces a bundle of analog pairs
- Your sensor measures more than one thing. A multi-parameter probe like a soil 5-in-1 sensor returns temperature, moisture, conductivity, salinity and TDS over a single connection; with 4–20mA that would need five separate loops
- You are logging data to a gateway, IoT platform or PC rather than a traditional PLC
- You want remote configuration and diagnostics instead of walking out to every sensor
You often don't have to choose just one system
Many industrial sensors are manufactured in both versions, so the decision is per measurement point, not per factory. For example, our temperature & humidity sensor (±0.3 °C, ±3 %RH) is available with RS485 Modbus or 4–20mA output, and the same is true of the atmospheric pressure sensor (300–1100 hPa, ±0.3 hPa). A common pattern: keep 4–20mA for a few critical standalone points, and put distributed environmental and soil sensors on a Modbus bus.
Rule of thumb: one sensor, one value, existing analog inputs → 4–20mA. Many sensors, multi-parameter data, digital logging → RS485 Modbus.
Browse our sensors by application in the Environmental Monitoring and Soil & Agriculture collections — every product page lists its available output options and includes a downloadable datasheet, so you can match the exact version to your controller before ordering.