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. 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. Star and ring topologies are not recommended because of signal reflections or excessively low or high termination impedance. Without termination resistors, signal reflections off the unterminated end of the cable can cause data corruption. Because a mark (logic 1) condition is traditionally represented (e.g. in RS-232) with a negative voltage and space (logic 0) represented with a positive one, A may be considered the non-inverting signal and B as inverting. Converters between RS-485 and RS-232 are available to allow a personal computer to communicate with remote devices. RS-485 is also used in building automation as the simple bus wiring and long cable length is ideal for joining remote devices. It requires minimal wiring and can share the wiring among several seats, reducing weight. 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, rs485 cable as can be seen in Profibus guides.

However this contradicts the facts that an idle mark state is a logical one and the termination polarization puts B at a higher voltage in Profibus guidelines. 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. Digital communications networks implementing the standard can be used effectively over long distances and in electrically noisy environments. In theatre and performance venues, RS-485 networks are used to control lighting and other systems using the DMX512 protocol. DH 485 is a proprietary communications protocol used by Allen-Bradley in their line of industrial control units. Although many applications use RS-485 signal levels, the speed, format, and protocol of the data transmission are not specified by RS-485. RS-485, also known as TIA-485(-A) or EIA-485, is a standard, originally introduced in 1983, defining the electrical characteristics of drivers and receivers for use in serial communications systems. RS-485 signals are used in a wide range of computer and automation systems. 12 V, i.e. ±7 V on top of the 0-5 V signal range. Failure to stay within this range will result in, at best, signal corruption, and, at worst, damage to connected devices.

This connection may be used to limit the common-mode signal that can be impressed on the receiver inputs. SC, G or reference, the common signal reference ground used by the receiver to measure the A and B voltages. It also defines three generator interface points (signal lines); A, B and C. The data is transmitted on A and B. C is a ground reference. Shield on communication cable is to remain continuous and ground only at the controller. 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. Interoperability of even similar devices from different manufacturers is not assured by compliance with the signal levels alone. These manufacturers all agree on the meaning of the standard, and their practice is in widespread use. Category 5 (Cat 5, Cat 5e, and Cat 6) cables are the most common Ethernet cables in use today. The most common cable type is Cat 5e-UTP (unshielded twisted pair) which may work over shorter distances in less demanding applications with low EMI noise levels.

Be sure to use a wire pair such as the blue and blue/white pair for the data lines and a third wire or pair twisted together for the common connection. RS-485 supports inexpensive local networks and multidrop communications links, using the same differential signaling over twisted pair as RS-422. By using repeaters very large RS-485 networks can be formed. Because they are produced in such large quantities, Cat 5 cables are relatively inexpensive, often less than half the price of specialty RS-485 cabling. These are used in programmable logic controllers and on factory floors. Utilizing a series of dedicated interface devices, it allows PCs and industrial controllers to communicate. This allows RS-485 to implement linear bus topologies using only two wires. If A is negative with respect to B, the state is binary 1. The reversed polarity (A positive with respect to B) is binary 0. The standard does not assign any logic function to the two states. Polarity of the wires must be observed. This section also defines the logic states 1 (off) and 0 (on), by the polarity between A and B terminals.