RS485, 4-20mA or 0-5V: How to Choose a Sensor Output Signal

The same temperature, humidity or pressure sensor can have different output forms, but they are oriented to completely different systems. If you select the wrong output mode, you may not be able to connect to the existing controller even if the sensor itself is normal at a later stage.

RS485: for multi-parameter, long-range and digital systems

RS485 adopts differential communication and strong anti-interference ability, which is suitable for long-distance wiring and multi-device bus. With Modbus RTU, an interface can poll multiple sensors by address and read digital quantities directly. It fits PLC, RTU, edge gateways, agricultural IoT and automated weather stations.

4-20mA: Suitable for industrial analog acquisition

4-20mA is a current signal without Modbus registers. After the collector reads the current, it is converted linearly according to the range. 4mA usually represents the lower limit of the range, and 20mA represents the upper limit of the range. Current loops are very common in industrial sites. The advantages are simple lines, good anti-interference ability, and "less than 4mA" itself can be a break or fault diagnosis clue.

0-5V: Simple and intuitive, but more dependent on line conditions

0-5V is a voltage analog. The controller must have a corresponding voltage input channel and be scaled according to the range. For long-distance wiring, voltage drop, ground potential difference, and electromagnetic interference generally have a more pronounced effect on voltage signals than current loops, and are therefore more suitable for systems with shorter distances and cleaner environments.

How to choose

• When a multi-device bus is required, multiple parameters are read, PLC/gateway is docked: RS485/Modbus RTU is preferred.



• If the existing control system has reserved 4-20mA AI channels, choose the current output version.
• For short cable runs where the controller only has voltage input: consider 0-5V.
• Do not interpret " 4-20mA output" as "can be read with Modbus", they belong to different signaling systems.

FAQ

Q1: Can 4-20mA be read through Modbus?

A1: A sensor with 4-20mA output does not output Modbus RTU data by itself. If the system requires Modbus, the conversion can be done by the acquisition module with analog inputs and Modbus outputs.

Q2: Why do I need multiple signal lines for the multi-parameter 4-20mA sensor?

A2: Because each parameter usually corresponds to an independent current output channel. For example, three analog inputs may be required for three parameters: temperature, humidity, and air pressure.

Q3: Does RS485 have to be more accurate than the 4-20mA?

A3: Accuracy depends first and foremost on the sensing element and the transmitting circuit. The digital transmission avoids the secondary conversion error of the analog quantity on the line, but it cannot simply equate the interface type with the sensor body accuracy.