RS485 and Modbus Explained for Sensor Buyers
RS485 is the wiring standard that carries the signal over long cable runs. Modbus RTU is the protocol that structures what's sent over that wiring. Together they let one data logger poll many sensors on a single two-wire bus.
What RS485 is
RS485 is a physical wiring standard: a two-wire differential signal that carries data reliably over long cable runs (up to roughly 1200 metres) and resists electrical noise better than simpler standards like RS232. It's a transport layer only, it doesn't define what the data means.
What Modbus adds
Modbus RTU is the protocol layer that runs over an RS485 connection. It defines how a data logger (the "master") requests specific readings from a sensor (the "slave") by register address, and how the sensor replies. This is why almost every sensor in this range lists "RS485 (Modbus RTU)" as its standard interface: RS485 carries the signal, Modbus RTU structures the conversation.
Why this combination dominates industrial sensors
Multiple RS485/Modbus sensors can share a single two-wire bus, each with a unique Modbus address, so one data logger or PLC can poll dozens of sensors on one cable run. That's why water quality installations in this range routinely combine several single-parameter probes (conductivity, pH, turbidity, dissolved oxygen) on one logger: they all speak the same protocol on the same bus.
Wiring basics
An RS485 bus needs two data wires (A/B or D+/D-) plus a common ground, wired in a daisy chain from sensor to sensor rather than a star topology. Long runs (over a few hundred metres) typically need a termination resistor at each end of the bus. Most sensors in this range also offer RS232, WiFi, Bluetooth or LTE as optional alternatives to RS485 where a wired bus isn't practical.
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Frequently asked questions
Can I run multiple sensors on one RS485 cable?+
Yes, that's the main advantage of RS485/Modbus. Each sensor is given a unique Modbus address, and many sensors can share the same two-wire bus and data logger, wired in a daisy chain rather than individually back to the logger.
What's the maximum cable length for RS485?+
RS485 can reliably run up to roughly 1200 metres at lower data rates, far beyond what RS232 or most wireless options achieve, which is part of why it's the standard for spread-out industrial and environmental monitoring sites.
Do I need Modbus RTU specifically, or will any RS485 device work with my logger?+
Your data logger or SCADA system needs to speak Modbus RTU (or you need a protocol converter) to read these sensors correctly, since RS485 alone is just the wiring, not the data format. Most industrial data loggers support Modbus RTU natively.