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Rs485 is low impedance balanced , so twisting the pair helps with noise immunity as well as reducing the attenuation effect of higher baud rates due to the parallel capacitance of the wires in the cable jacket over the long lengths. RS-485 supports inexpensive local networks and multidrop communications links, using the same differential signaling over twisted pair as RS-422. The cable to be used is a shielded twisted pair (telephone type). Each device has a communication port with two terminals, which are indicated for the sake of convenience as A and B. In these two terminals the communication cable is connected so that all the devices that take part in the communication are connected in parallel. Star and ring topologies are not recommended because of signal reflections or excessively low or high termination impedance. Interoperability of even similar devices from different manufacturers is not assured by compliance with the signal levels alone. Most manufacturers still utilize a DB-9 connector. These manufacturers all agree on the meaning of the standard, and their practice is in widespread use. In a computer system, SCSI-2 and SCSI-3 may use this specification to implement the physical layer for data transmission between a controller and a disk drive.

This allows RS-485 to implement linear bus topologies using only two wires. RS-485 is used as the physical layer underlying many standard and proprietary automation protocols used to implement industrial control systems, including the most common versions of Modbus and Profibus. RS-485 standard conformant drivers provide a differential output of a minimum 1.5 V across a 54-Ω load, whereas standard conformant receivers detect a differential input down to 200 mV. Multiple receivers may be connected to such a network in a linear, multidrop bus. The recommended arrangement of the wires is as a connected series of point-to-point (multidropped) nodes, i.e. a line or bus, not a star, ring, or multiply connected network. A simple network of a PLC, VFD, and an HMI allows remote control of motors in an industrial setting. This is possible since the HMI is connected to your PLC via RS485. All in all, RS485 really allowed full automation possible with its ability to connect so many devices at once. The RS485 system used for Modbus communication provides a main cable (Bus or backbone), to which all the devices have to be connected with branches (also known as stubs) that are as short as possible.

The termination also includes pull up and pull down resistors to establish fail-safe bias for each data wire for the case when the lines are not being driven by any device. With Modbus, BACnet and Profibus, A/B labeling refers A as the negative green wire and B as the positive red wire, in the definition of the D-sub connector and M12 circular connector, as can be seen in Profibus guides. Line A voltage, implying A, the green wire, is indeed connected to the driver inverting signal, as seen in a whitepaper. Section 4 defines the electrical characteristics of the generator (transmitter or driver), receiver, transceiver, and system. In contrast to RS-422, which has a driver circuit which cannot be switched off, RS-485 drivers use three-state logic allowing individual transmitters to be deactivated. In theatre and performance venues, RS-485 networks are used to control lighting and other systems using the DMX512 protocol. If a star configuration is unavoidable, special RS-485 repeaters are available which bidirectionally listen for data on each span and then retransmit the data onto all other spans.

The equipment located along a set of RS-485 wires are interchangeably called nodes, stations or devices. Converters between RS-485 and RS-232 are available to allow a personal computer to communicate with remote devices. This robustness is the main reason why RS-485 is well suited for long-distance networking in noisy environment. The maximum number of devices that can be connected to a main cable is 32, including the Master. Without termination resistors, signal reflections off the unterminated end of the cable can cause data corruption. As a rule of thumb, the speed in bit/s multiplied by the length in metres should not exceed 108. Thus a 50-meter cable should not signal faster than 2 Mbit/s. RS-485 is also used in building automation as the simple bus wiring and long cable length is ideal for joining remote devices. Using a repeater, the main cable is divided into different segments, each of which can be up to 700 m in length and connect 32 devices (this number includes the repeaters). Normally, this connection is made at one end of the main cable. The end resistance must be used only at the ends of the main cable.

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