Technical data

www.westermo.com Theoretical and general applications 29
Industrial interfaces
RS-422
RS-422 is an ideal standard for industry as the interface is created to build data buses,
typically multidrop, between central computers and a number of substations. The
interface is balanced and relatively insensitive to interference. The interface switches
polarity on the wire pair depending on whether it is a one or a zero being
transferred. The original specification for RS-422 states that communications can take
place from one master to 10 slaves, which can only listen to the traffic. We use the
drive circuits for RS-485, where the transmitter can communicate with 32 units and
can be operated to “tri-state”, which means we can design applications with multidrop
over both 4-wire and 2-wire connections.
The recommended maximum distance is 1200 m (4000 ft) at a transmission rate of
100 kbit/s. The drive circuits support data rates up to 10 Mbit/s, but the transmission
range then drops to 20 m (66 ft). RS-422 can be integrated with RS-485, RS-232/
V.24 by using a converter.
RS-422 on 4-wire
In an RS-422 4-wire system the master transmitter can
always be active/switched on, depending on the activity of
the slaves. The standard permits simultaneous duplex
communications.
RS-485
RS-485 is a further development of RS-422 and is
increasingly used as standard on different equipment.
The greatest advantage of RS-485 is that it supports
2-wire communications, i.e. the transmitter and receiver in the equipment can switch
the direction of communication. It is designed for data buses of up to 32 devices and is
suitable for multidrop networks where a master/slave relation is employed. The re-
commended maximum range is 1200 m (4000 ft) with a transmission
rate of 100 kbit/s. there are many different standard inter-
faces that use RS-485 as its physical media, for example,
PROFIBUS, Interbus-S and Bitbus.
Meter
10 kb/s
10 000
1 000
100
10
1 200
100 kb/s 1 mb/s 10 mb/s
TD
RD
RTS
DTR
SG
TD
RD
RTS
DTR
SG
TX A
TX B
RX A
RX B
RX A
RX B
TX A
TX B
RS-232
DEVICE
RS-422
DEVICE
TD
RD
RTS
DTR
SG
TD
RD
RTS
DTR
SG
TX A
TX B
RX A
RX B
RS-422
DEVICE
RS-422
DEVICE
RS-422
DEVICE
RS-422
DEVICE
TD
RD
RTS
DTR
SG
TD
RD
RTS
DTR
SG
A
B
RS-485
DEVICE
RS-485
DEVICE
RS-485
DEVICE
RS-232
DEVICE
Termination
RS-485 communication
distance
B A C K