Connecting RS-422/485 Devices
18
GND
GND
TXA TXB RXA RXB
RS-422/485
TXA TXB RXA RXB
RS-422/485
GND
Figure 3-1: RS-422/485 point-to-point connection
Notice the termination on each end of the connection. The receiving end of the wire should
be terminated with a resistance equal to the wire's characteristic impedance, generally 100-
120 ohms.
RS-422/485 Bus Configurations
More than two RS-422/485 devices can be connected in a bus configuration. You can connect
one RS-422 and several RS-485 devices on a bus or you can connect several RS-485 devices
on a bus, but you cannot connect more than one RS-422 device on a single bus. If your
configuration includes an RS-422 device, you must use a four-wire connection.
Configurations including only RS-485 devices can use either two-wire or a four-wire
communication.
When RS-422 and/or RS-485 devices are connected to a bus, they operate as one master and
one or more slaves. In all configurations that include an RS-422 device, the RS-422 device is
the master and the RS-485 devices are slaves.
Two-Wire System
In two-wire communication, all devices share the same pair of wires to both transmit and
receive. All the devices connected to a two-wire system must be RS-485 devices. Figure 3-2
shows a typical two-wire system.
