An RS485/RS422 network consists of many different components,
such as cables, RS232/RS485/RS422 devices (master and slaves),
converters, and power supplies, and because this type of
network is normally
used over a long distance and in an electrically noisy
environment, noises, reflections, and data errors might
occur. Building a reliable RS485/RS422 network by choosing the
correct cables, designs, and converters could save a lot of
valuable engineering time and make the system free of the
above-mentioned problems.
RS485/RS422 Topology
RS485/RS422 is a
network in a linear, multi-drop configuration;
however, star-wiring is inevitable in some
places. Our RS485 Hub/Splitter
(Model:
HUB-485-4) splits an RS485 network into four, thus making star-wiring possible.
RS485/RS422 Distance
A single RS485/RS422
loop can go up to 4000ft (1200m) by using standard copper wire (screened twisted pair).
This
distance (using copper wire) can be extended to another 4000ft
(1200m) by using our RS485/RS422 repeater
(Model:
RPT-485_422-2 and
RPT-485_422-4). Note:
A maximum of two repeaters can be used in
series.
The RS485/RS422 distance can
be further extended to up to 24.8 miles (40km)
by using Single-Mode Fiber Optic Cables and our Single-Mode RS232/485/422 to Fiber
Optic Media converter (Model:
FBR-Serial-2).
RS485/RS422 Cabling and
Termination
Screened twisted-pair cable (e.g. CAT-5e) is
recommended to prevent external noises induced
to the RS485/RS422 buses. Transmission lines are usually terminated to
reduce noises and reflections that can cause data errors.
Both ends of the bus
should be terminated (not each node). See illustration
below.
Notes:
1) Termination is important for impedance
matching and noise / reflection reduction,
therefore, it is required regardless of whether
port- or externally-powered converters are used.
2) Terminator
adds heavy DC loading to a system, and
therefore, connect it only when the data rate is
over 19.2kbps or the RS485/RS422's
distance exceeds 660ft (200m). 3) CommFront's
built-in 120-Ohm terminators can be easily
turned ON or OFF through a simple jumper
setting, thus making our converters ideal
solutions for all types of installations.
RS232/RS485/RS422 Converters
1) Industrial- or
Commercial-grade?
Just as there are many differences
between industrial- and commercial-grade computers, the industrial-
and commercial-grade converters differ in many ways, such as the
level of circuit protection, temperature range
and noise tolerance.
Most importantly, true industrial-grade converters require rugged
components and reliability-engineering designs
to withstand heat, vibration, shock and other
environmental factors, therefore, they are much more durable
than commercial-grade
converters. Industrial-grade converters are designed to interface
with mission-critical equipment, such as computers, PLC, SCADA and HMI - whether in the field, server room, factory, control room, or
fire command center.
2) Port- or Externally-powered?
Most of the RS232 ports come with a -3 to -15VDC
port power, and that's the reason why
port-powered converters are very popular on the
market. However, because the port-powered converters require a
minimum of +/-5VDC from the RS232 port, there may be insufficient port power, which could cause data errors.
Some of our port-powered converters
accept external 5VDC input, which solves this problem
effectively. And in
case there is no port power at all from the
RS232 port, the externally-powered converter
(Model:
CVT-485_422-4) can be used.
3) Non-isolated or
Opto-isolated?
Opto-isolation uses light instead of electrical signals to
transmit data. It isolates the RS232 and RS485
devices from each other, thereby effectively protecting RS232 devices from
electrical spikes or surges induced by RS485 networks.
Furthermore, it reduces noise and
eliminates the ground loop problem. If your
RS232 or RS485/RS422 devices are generating a lot
of noise, try the opto-isolated converters,
which solve the problem most of the time.
4) 120-Ohm
end-of-line terminators
There are several methods of terminating data
lines, and we recommend parallel termination. Instead of using pull-up or pull-down resistors, the
parallel 120-Ohm end-of-line terminators provide a
hassle-free and effective way of termination.
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