Systech Corporation NDS/5000, Hardware Manual

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Using the NDS in RS-422 or RS-485 Mode 26
Four-Wire Systems
In four-wire communication, there are two pairs of transmit and receive lines, allowing full
duplex communication. In most four-wire systems, an RS-422 device will serve as the master
with several RS-485 devices as slaves. However, an RS-422 device is not required; an RS-485
device can serve as the master. Figure 3-3 shows a typical four-wire system.
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 3-3: Four-wire RS-422/485 System
In most configurations, the device that is designated as the master will be an RS-422 device
that constantly drives the transmit lines.
Both pairs of lines on 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 from other lines on this serial port, or from other nearby ports, may be coupled
back onto the unterminated receive lines. You should attach a simple 100- or 120-ohm
termination resistor between the lines to avoid this.
Using the NDS in RS-422 or RS-485 Mode
The NDS ports can function as any of the devices in any of the configurations described
above. The NDS can be connected to either an RS-422 or an RS-485 device in a point-to-point
configuration. Or the NDS 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
NDS is connected to the bus and functions as the master, but this is determined by the
application. The ports are configured for RS-232, RS-485 or RS-422 via software. Refer to the
NDS Administrator’s Guide.