RS-422 RS-485

RS-422 and RS-485 Electronic Design & Firmware
by Embedded Design Engineer John Heritage

I have worked with the RS-485 protocol when designing DMX lighting control hardware, by using differential line drivers high speed serial data can be achieved in a multi node control system.

TIA/EIA-422-B (typically referred to as RS-422) and TIA/EIA-485-A (typically referred to as RS-485) are balanced, twisted-pair interfaces capable of speeds up to 10Mbps and distances up to 4,000 feet. Being differential buses, each uses signals from 1.5V to 6V to transmit the data. (With a differential, balanced bus, noise immunity is increased over a comparable single-ended, unbalanced bus such as RS-232.)

The RS-422 interface is a multi-drop interface, giving unidirectional communication over a pair of wires from one transmitter to several receivers, up to 10 unit loads (UL). If the devices receiving the data wish to communicate back to the transmitter, the designer must use a separate, dedicated bus between each receiver and the transmitter. (Using this return bus will allow full-duplex transmissions.) For that reason, RS-422 is seldom used between more than two nodes.

The RS-485 interface, on the other hand, is a bidirectional communication over one pair of wires between several transceivers. The specification states that the bus can include up to 32 UL worth of transceivers. Many manufacturers produce fractional-UL transceivers, thereby increasing the maximum number of devices to well over 100.

The RS-422 and RS-485 interfaces often use the same start bit/data/stop bit format of RS-232. In fact, several converters exist to go from RS-232 to RS-485 and back. Do keep in mind, however, that RS-232 is a full-duplex interface, while RS-485 is half-duplex.

Several microcontroller manufacturers provide built-in UARTs that boast special RS-485 abilities.