User Manual

Ports RS-485 Configuration
8-17
8.10.2 Four-wire Mode
In four-wire mode, the SCS operates in full duplex: one pair of wires functions as the transmit pair, another
pair of wires functions as the receive pair, and there is a shield/ground wire for each pair. The SCS is able
to send and receive data simultaneously. In a four-wire RS-485 network, one device acts as a master while
the other devices are slaves. The advantages of four-wire mode are double the throughput of two-wire mode
and a guaranteed open path to each slave device’s receiver.
Figure 8-42: Example Four-Wire Mode Network
It is important to connect the transmitter of the master device to the wire that is connected to the receive
terminals on the slave devices, and connect the receiver of the master device to the wire that is connected
to the transmit terminals on the slave devices. In essence, the master device will be connected to the slave
devices with a swapped cable.
Figure 8-43: Enabling Four-Wire RS-485 Mode
8.10.2.1 TXDrive
The SCS can be configured to either always drive the TX (transmit) signal or to let the attached device
control the TX signal (tristate) when not actively transmitting. The Define Protocols RS485 TXDrive
command takes one of two parameters. The Always parameter sets the SCS for continuous TXDrive, so TX
will never be tristated. The Auto parameter sets the SCS for TXDrive when transmitting and tristate while
idle.
Figure 8-44: Changing TXDrive
Note: You can only set TXDrive for Always when using four-wire mode. The Always
parameter returns an error in two-wire mode.
Local>> DEFINE PROTOCOLS RS485 MODE 4WIRE
Local>> DEFINE PROTOCOLS RS485 TXDRIVE AUTO
TX
RX
RX
TX
TX-
TX+
RX+
RX-
Shield
Shield
RX-
RX+
TX+
TX-
Shield
Shield
Sh
-
-
+
+
TX
RX
Sh
Sh
-
-
+
+
TX
RX
Sh
Sh
-
-
+
+
TX
RX
Sh
Master
(SCS)
Slave Slave Slave
Slave