RS485 (also known as EIA-485) is a two-wire, half-duplex serial communications link. It is the standard most commonly used for industrial applications due to its high noise immunity and multi-drop capability. Up to 128 devices can communicate via RS485, over distances up to 1200 metres using inexpensive twisted pair wires.
Data speeds can be as high as 35 Mbit/s over 10 m and 100 kbit/s at 1200 m, but many devices do not support such high data rates. RS485 uses differential signals (the voltage difference between the wires) to convey data.
One polarity indicates a logic 1, the reverse polarity indicates logic 0. The applied voltages can be between +12 V and -7 volts, but the difference of potential need only be >0.2 volts for valid operation.
It is recommended that the wires be connected as series of point-to-point (multi-dropped) nodes (not in a star or ring format), with 120ohm termination resistors connected across the wires at the two ends of the network. Without termination resistors, reflections of the signals can cause data corruption, and electrical noise sensitivity is increased.
The master device should normally provide powered resistors, to bias the wires to known voltages when they are not being driven by any device. Without biasing resistors, the data lines float and noise can be interpreted as data when actually all devices are silent.
RS485 only defines the physical layer electrical specification, not the protocol that is transmitted across it. Various protocols are possible, but the most common protocol for industrial instrumentation is Modbus RTU.
HMIs PLCs and other instruments frequently have RS485 ports but PC’s do not. To connect an RS485 network to a PC USB to RS485 or RS232 to RS485 convertor will be required