What Is RS485? Industrial Serial Communication Basics & Practical Wiring

RS485 (TIA/EIA-485-A) is a widely used physical-layer serial communication standard for industrial environments. It adopts differential signal transmission over a twisted pair, using the voltage difference between line A and line B to transmit data, instead of single-ended voltage relative to ground. This design strongly suppresses common-mode noise and makes it much more resistant to electromagnetic interference than RS232.

Standard 2-wire RS485 works in half-duplex mode: devices cannot send and receive simultaneously. One bus can connect up to 32 nodes without repeaters, supporting long-distance transmission of roughly 1200 meters at low baud rates; higher speeds shorten the maximum cable length significantly. For reliable communication, 120-ohm terminating resistors must be installed at the two physical ends of the bus to eliminate signal reflection; middle nodes should not add terminators.

RS485 only defines electrical levels and does not include an upper-layer protocol. In power equipment, it almost always runs with Modbus RTU. Charging piles, UPS systems, battery monitors and energy meters use RS485 to report voltage, current, temperature, load rate and fault codes to PLCs or gateways.

A frequent confusion is mixing RS485 with dry contacts. Dry contacts carry only on/off status and cannot transfer numerical data; RS485 continuously transmits digital measurement values and parameters. Common field faults include reversed A/B wiring, star topology, missing terminators and large ground potential differences. Adding isolated RS485 modules helps avoid damage in noisy industrial sites.