Introduction
RS-485 Networking Overview
1 - 3
On a two-wire RS-485 network, the MSS must turn its transmitter on
when it is ready to send data and then off a certain period of time after
the data has been sent so that the line is available to receive again. At
most baud rate settings, the timing delay is typically one character length
with a maximum of 1.5 character lengths.
NOTE:
For 600 baud and 4800 baud operation, the timing delay is
doubled.
1.1.3 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. In a four-
wire RS-485 network, one device, such as the MSS, acts as master while
the other devices are slaves.
Figure 1-2:
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.
The MSS transmitter is always driving and the receiver is always enabled,
so the MSS is able to send and receive data simultaneously. 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.
TX
RX
RX
TX
TXA
TXB
RXB
RXA
Shield
Shield
TXA
TXB
RXB
RXA
Shield
Shield
Sh
A
A
B
B
TX
RX
Sh
•
•
•
•
•
•
Sh
A
A
B
B
TX
RX
Sh
•
•
•
•
•
•
Sh
A
A
B
B
TX
RX
Sh
•
•
•
•
•
•
MSS
Slave
Slave
Slave
Slave
(Master)