Serial Communication Adapters Instruction Manual
6
Serial Communications
Overview
RS-232 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 9illustrates how to connect a general four-wire
network (a four-wire network actually contains 5 wires).
Figure 9. General Four-Wire Network Connections
Figure 10 illustrates how to connect a general two-wire
network (a two-wire network actually contains 3 wires).
Figure 10. General Two-Wire Network Connections
