Lantronix MSS4-D, Installation Manual

Page: 38 / 95

RS-485 Configuration Configuration
4- 8
4.3.2 Four-wire Mode
In four-wire mode, the MSS 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 MSS 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 4-11: 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 4-12: Enabling Four-Wire RS-485 Mode
4.3.2.1 TXDrive
The MSS 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
Change
RS485 TXDrive command takes one of two parameters. The Always parameter sets the
MSS for continuous TXDrive, so TX will never be tristated. The Auto parameter sets the
MSS for TXDrive when transmitting and tristate while idle.
Figure 4-13: Changing TXDrive
Note:
You can only set TXDrive for Always when using four-wire mode. The
Always parameter has no effect for two-wire mode.
Local>> CHANGE RS485 PORT 3 MODE 4WIRE
Local>> CHANGE RS485 PORT 3 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
(MSS)
Slave Slave Slave
Slave