Serial Communication Converters & Adapters Instruction Manual
6
Serial Communications
Overview
RS-232, RS-422, and RS-485 are standard
interfaces approved by the Electronic Industries
Alliance (EIA) for connecting serial devices. In
EIA terms, the device (e.g. meter) that connects
to the interface is called a Data Communications
Equipment (DCE) and the device to which it
connects (e.g. the computer) is called a Data
Terminal Equipment (DTE).
The RS-422 standard was designed to replace
the older RS-232 standard because it supports
higher data rates and greater immunity to
electrical interference. RS-485 is similar to RS-
422 but can support multi-point connections per
line because it uses lower-impedance drivers
and receivers.
Line drivers and receivers are used to exchange
data between two or more points (nodes) on a
serial communications network. Reliable data
communications can be difficult in the presence
of induced noise, ground level differences, and
other hazards associated with installation of a
network. When communicating at high data
rates, or over long distances in real world
environments, RS-232 is often inadequate. The
differential data transmission of RS-422 and RS-
485 offers superior performance in most
applications. Differential signals can help nullify
the effects of ground shifts and induced noise
signals that can appear as common mode
voltages on a network.
RS-422 was designed for greater distances and
higher baud rates than RS-232. In its simplest
form, a pair of converters from RS-232 to
RS-422 (and back again) can be used to form
an "RS-232 extension cord". Data rates of up to
100 kbits/second and distances of 3,937' (1,200
m) can be accommodated with RS-422.
RS-422 devices however cannot be used to
construct a true multi-point network. A multi-
point network consists of multiple drivers and
receivers connected on a single bus, where any
point (node) can transmit and/or receive data.
RS-485 is an enhanced version of the RS-422
standard, which allows multiple drivers and
receivers on the same two-wire or four-wire
system. The RS-485 standard specifies up to 32
drivers and 32 receivers on a single bus, but
with the introduction of "automatic" repeaters
and high-impedance drivers/receivers, this
number can be extended to hundreds of points
(nodes) on a network.
The cabling used for an RS-422 or RS-485
serial communications network should always
be a high quality cable such as Belden 8162 or
Alpha 6203C. A two-wire system requires two
twisted pairs, and a four-wire system requires
three twisted pairs (the extra twisted pair is
needed for the signal ground).
Figure 1 illustrates how to connect a general
four-wire network (a four-wire network actually
contains 5 wires).
PDA8485
DO
DO
DI
DI
GND
DO
DO
DI
DI
GND
RS-422/485 DEVICE
Figure 1. General Four-Wire Network
Connection
Figure 2 illustrates how to connect a general
two-wire network (a two-wire network actually
contains 3 wires). Note that the PDA7485 and
PDA8485 have DIP switches that allow for two-
wire connections without the need to externally
wire the DO to the DI and the /DO to the /DI (see
the converter section for complete details).
PDA8485
DATA
DATA
DO
DO
DI
DI
GND
GND
RS-485 DEVICE
Figure 2. General Two-Wire Network
Connection
