Information Technology Reference
In-Depth Information
14
RS-422, RS-423 and RS-485
14.1 Introduction
The main standards organisations for data communications are the ITU (International Tele-
communications Union), the EIA (Electronic Industry Association) and the ISO (Interna-
tional Standards Organisation). The ITU standards related to serial communications are de-
fined in the V-series specifications and EIA standards as the RS-series. The EIA has defined
many standards for serial communications. RS-232 has many limitations, such as:
•
One transmitter and one receiver.
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Maximum connection length of 20 m.
•
Maximum baud rate of 20 kbps.
The RS-422 and RS-423 standards replace the RS-232 standards and support higher data
rates and greater immunity to electrical interference. The main standards are:
•
RS-422A - Supports multipoint connections. It defines the electrical characteristics of
balanced load voltage digital interface circuits.
•
RS-423A - Supports only point-to-point connections. It defines electrical characteristics
of unbalanced voltage digital interface circuits.
•
RS-449 - Defines the basic interface standards and refers to the RS-422/3 standards. It
defines a general-purpose 37-position and 9-position interface for DTE and DCE em-
ploying serial binary data interchange.
•
RS-485 - Similar to RS-422 but can support more nodes per line because it uses lower-
impedance drivers and receivers.
14.2 RS-485 (ISO 8482)
RS-485 is an upgraded version of RS-422 and extends the number of peripherals that can be
interfaced. It allows for bidirectional multipoint party line communications. This can be used
in networking applications. RS-422 and RS-232 facilitate simplex communication, whereas
RS-485 allows for multiple receivers on a single line, facilitating half-duplex communica-
tions. The maximum data rate is unlimited and is set by the rise time of the pulses, but it is
usually limited to 10 Mbps. A network using the RS-485 standard can have up to 32 transmit-
ters/receivers with a maximum cable length of 1.2 km, as shown in Figure 14.1. The maxi-
mum cable length is 1200 m.
