Systech Corporation RCS/3000, Hardware Manual

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16 Connecting RS-422/485 Devices
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 10-1
shows a typical two-wire system.
GND
GND
TXA TXB RXA RXB
RS-485
MASTER
TXA TXB RXA RXB
RS-485
TXA TXB RXA RXB
RS-485
GND
GND
SLAVE
SLAVE
Figure 10-2. Two-wire RS-485 System
In the two-wire system diagrammed in Figure 10-2, one pair of transmit and receive lines
(TXA and RXA) are connected to a single wire and the other pair of transmit and receive
lines (TXB and RXB) are connected to the second wire. The device that is designated as the
master manages the traffic on the lines.
As shown in the above diagram, the pair of transmit/receive lines needs to be terminated
with 120 ohms at each end of the bus.
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 10-3 shows a typical four-wire system.