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.

Digital communications networks implementing the standard can be used effectively over long distances and in electrically noisy environments.

RS-485 supports inexpensive local networks and multidrop communications links, using the same differential signaling over twisted pair as RS-422.

Star and ring topologies are not recommended because of signal reflections or excessively low or high termination impedance.

[further explanation needed] The value of each termination resistor should be equal to the cable characteristic impedance (typically, 120 ohms for twisted pairs).

The EIA has officially disbanded and the standard is now maintained by the TIA as TIA-485, but engineers and applications guides continue to use the RS-485 designation.

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.

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 is also used in building automation as the simple bus wiring and long cable length is ideal for joining remote devices.

The RS-485 standard states (paraphrased):[14] The truth tables of most popular devices, starting with the SN75176, show the output signals inverted.

The two values provide a sufficient margin for a reliable data transmission even under severe signal degradation across the cable and connectors.

[28] In addition to the A and B connections, an optional, third connection may be present (the TIA standard requires the presence of a common return path between all circuit grounds along the balanced line for proper operation)[29] called SC, G or reference, the common signal reference ground used by the receiver to measure the A and B voltages.

Grounds between buildings may vary by a small voltage, but with very low impedance and hence the possibility of catastrophic currents – enough to melt signal cables, PCB traces, and transceiver devices.

The standard does not discuss cable shielding but makes some recommendations on preferred methods of interconnecting the signal reference common and equipment case grounds.

The diagram below shows potentials of the A (blue) and B (red) pins of an RS-485 line before, during, and after transmission of one byte (0xD3, least significant bit first) of data using an asynchronous start-stop method.