Systech Corporation RCS/3000, Hardware Manual

Page: 23 / 32

Physical Layer Interface Settings 17
GND
GND
TXA TXB RXA RXB
RS-422/485
MASTER
TXA TXB RXA RXB
RS-485
TXA TXB RXA RXB
RS-485
GND
GND
SLAVE
SLAVE
Figure 10-3. Four-wire RS-422/485 System
In most configurations, the device that is designated as the master will be an RS-422 device
to constantly drive the transmit lines.
Both pairs of lines of the multi-drop wire must be terminated at each end of the bus with a
resistance equal to the wire's characteristic impedance, generally 120 ohms. If the RX data
pins are not terminated at all and have nothing attached, they may be susceptible to cross
talk. Data on other lines on this port or other nearby ports may get coupled back onto the
unterminated receive lines. If you need to leave the RX lines unattached, you may need to
attach a simple 100 or 120 ohm termination resistor between the lines to avoid this.
Using the RCS/3000 in RS-422 or RS-485 Mode
Any serial port on the RCS/3000 can be configured for either RS-422 or RS-485
communication. Thus, the RCS/3000 can function as any of the devices in any of the
configurations described above. The RCS can be connected to either an RS-422 or an RS-485
device in a point-to-point connection. Or an RCS can be connected to a two-wire or four-
wire bus, as either a master or a slave, and can communicate in either RS-422 or RS-485
mode. Typically, a single RCS/3000 is connected to the bus and functions as the master, but
this is determined by the application.
Connecting the RCS/3000
Connect the RCS/3000 to the network using any serial port. The DB-9 pinouts for the
RCS/3000 in RS-422 and RS-485 modes are: