Systech Corporation RCS/5000, Hardware Manual

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Physical Layer Interface Settings 25
RS-422/485 Point-to-point Configuration
Point-to-point connections can be established between two RS-422 devices, an RS-422 and an
RS-485 device, or two RS-485 devices. Figure 3-1 shows a four-wire, point-to-point
connection between two devices.
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
ohms.
RS-422/485 Bus Configurations
More than two RS-422 and RS-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.