What Is RS-485?

RS-485 is an electrical standard for serial data, published as TIA/EIA-485-A. It defines how the signal travels on the wires, not what the data means, so protocols such as Modbus RTU, Profibus DP, BACnet MS/TP and Pelco-D run on top of it. RS stood for Recommended Standard.

Instead of measuring a signal against ground, as RS-232 does, RS-485 sends each bit as the difference in voltage between two wires, usually labeled A and B or D− and D+. The receiver only looks at that difference. Electrical noise from motors, welders or power cables tends to hit both wires equally, so it cancels out. A difference of just 200 mV is enough for the receiver to read a valid bit, and the standard tolerates a common-mode voltage range of −7 V to +12 V between devices. That is why RS-485 survives long runs through factories, vehicles and outdoor installations where RS-232 would fail.

RS-485 is also multi-drop. Many transmitters and receivers can share the same pair of wires, so a single bus can link a controller to dozens of meters, drives or sensors.

Distance and speed trade off against each other. A common rule of thumb is that the data rate in bit/s multiplied by the cable length in meters should stay below about 10⁸, and slower rates such as 9,600 or 19,200 bit/s are typical on long industrial buses.

Most RS-485 faults come from wiring rather than from the equipment. A few rules keep a bus reliable.

Termination and biasing solve two different problems. A termination resistor matches the end of the cable to its characteristic impedance, so a signal that reaches the end is absorbed instead of reflecting back along the line and corrupting the next bits. Fit a resistor close to the cable impedance, usually 120 ohm, across A and B at the two ends of the trunk only. Extra terminators part-way along the bus load the drivers and shrink the signal.

Reflections matter most on long cables and at higher baud rates. On a few meters of cable at 9,600 baud they die away long before the receiver samples each bit, which is why an unterminated bench test can work and the same bus then fails on site. Terminating both ends is the safe default.

Biasing, also called fail-safe biasing, deals with the idle bus. Between messages every driver is switched off, and an undriven pair floats near 0 V, inside the receivers’ ±200 mV undefined band, so noise can be read as a start bit. A pull-up resistor on the non-inverting line and a pull-down on the inverting line hold the pair at least 200 mV apart, in the idle state. Typical values are a few hundred ohms, such as 560 or 680 ohm on a 5 V supply, fitted once per bus, often at the controller. Many current transceivers have fail-safe receivers that read an idle or shorted bus as a logic 1, which reduces or removes the need for external bias, so check the datasheet before adding resistors.

Watch the labels too. TIA-485 calls the inverting line A and the non-inverting line B, but many devices print them the other way round or use D+ and D−, so a bus that stays silent after wiring often has one pair swapped.

RS-422 is the close relative of RS-485. Both use differential signaling over twisted pairs and reach similar distances and speeds. The difference is the number of transmitters. RS-422 allows one driver and up to 10 receivers on a pair, so it suits point-to-point or one-to-many links, such as a computer sending data to several displays. RS-485 allows many drivers on the same pair, each switching off when it is not transmitting, which is what makes a shared multi-drop bus possible. Most RS-485 transceivers can also talk to RS-422 equipment in four-wire mode, which is why industrial COM ports usually offer both.

RS-485 remains the default physical layer wherever simple, robust links run long distances between many devices.

Industrial computers usually offer RS-485 as one mode of a combined RS-232/422/485 COM port, selected in the BIOS or with jumpers. For harsh sites, look for electrical isolation, which protects the computer from ground loops and surges, ESD protection and automatic direction control, which switches the transceiver between sending and receiving without software timing. Rugged and vehicle computers often carry serial on locking M12 connectors; our M12 A-coded FAQ covers the connector usually used. Where more ports are needed, our serial I/O modules add isolated RS-485 channels on mini PCIe or PC/104 cards. In our object tracking case study, an edge AI computer uses RS-485 to steer a PTZ camera.

We specify and fit isolated RS-485 ports on industrial and edge computers as standard, set the port modes and termination for your bus, and test serial communication before each system ships.