Serial Interfaces RS232, RS422, RS485

Seria interfaces

Application Areas, Versions, Converters

Serial interfaces generally and in colloquial terms refer to data transmissions where the data bits are transferred chronologically one after the other over a logical line. In particular, the term "serial interfaces" is often used for RS232, RS422 and RS484.

Since the increased implementation of USB and Ethernet, RS232 (as well as the parallel printer interface) has lost importance in the PC as a communication interface for general peripherals. However, serial interfaces according to RS232, RS422 and RS485 are still used today:

Application Area

Application area RS232: Configuration and service connections for example of switches and routers, laboratory devices and POS terminals (Point-Of-Sales).
Application area RS485: Industrial field communication, e.g. basis for some field buses (PROFIBUS, MODBUS).

A simple, obvious solution to still be able to use RS232 or RS422/485 in modern PCs without serial interfaces is to convert USB to RS232 or RS422/485.

Some basic characteristics apply to serial data transmission:

Transmission Mode

Simplex, directional mode: Data can be transmitted in only one direction.
Half-duplex, alternating mode: Data can flow alternately, but not simultaneously, in both directions.
Full duplex, reverse mode: Data can be transmitted in both directions simultaneously.

Data Structure

Start bit marks the beginning of a transmission.
Data bits contain the actual user data.
Parity bit is used for parity checking/error detection; represents "even", "odd", "mark", "space" or "none".
Stop bit marks the end of the message transmission.

Transmission Rate

Bit rate: Data transmission rate in bits per second (bit/s or bps).
Baud rate: Data transmission rate in symbols per second (baud, Bd). Depending on the coding, a symbol can comprise different numbers of bits of a data stream.

In Comparison: RS232, RS422 and RS485 Key Features

Standard	Transmission	max. rate and distance	Lines	Topology	Max. devices
RS232	Full-duplex	Not specified, up to 115.2 kbit/s and more/15 m	Max. 9: TxD, RxD, RTS, CTS, DTR, DSR, DCD, GND	Point-to-point	1
RS422	Full-duplex	10 Mbps/1.2 km	Max. 5: TxD+, TxD-, RxD+, RxD-, GND	Point-to-point	1 (10 devices in receive mode)
RS485	Half- or full-duplex (2 or 4 wires)	12 Mbps/1.2 km	Max. 5: TxD+, TxD-, RxD+, RxD-, GND	Multi-point	32/256 with repeaters

Data and Control/Handshake Lines

TxD, RxD, for RS422/485 differential as TxD+/- and RxD+/-: Transmit and receive data lines, lines for user data.
RTS - Request to Send and CTS - Clear to Send.
DTR - Data Terminal Ready.
DSR - Data Set Ready.
(D)CD - (Data) Carrier Detect.
RI - Ring Indicator.
GND - Ground Line (Ground).