Lantronix Device Server MSS 485 Installation Manual - 1.1.1 Rs-485 Protocols
RS-485 Networking Overview
Introduction
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1.1 RS-485 Networking Overview
The RS-485 standard allows a transmission line to be shared like a “party
line.” As many as 32 devices can share the multidrop network. Typically,
one device is the master and the other devices are slaves. There are a few
important things to note about RS-485 networking with the MSS.
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The MSS can be used in either two-wire or four-wire mode. Refer
to the following sections to determine which mode to use.
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The maximum RS-485 network cabling length (without repeaters)
is 4,000 feet. Lantronix recommends the use of shielded twisted-
pair cabling.
1.1.1 RS-485 Protocols
A large number and variety of protocols run over RS-485. However, the
MSS does not convert or interpret serial data. It only moves data between
serial and Ethernet. Any RS-485 protocol will have to be implemented by
host software.
1.1.2 Two-wire Mode
In two-wire mode, the MSS operates in half duplex: one pair of wires
shares transmit and receive signals, and an optional third wire can be
used for shield/ground. The main advantage of using two-wire mode is
reduced cabling costs.
Figure 1-1:
Example Two-wire Mode Network
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TXA
TXB
TX
RX
Shield
A
A
B
B
A
B
B
A
RX
TX
Shield
Shield
Shield
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A
B
A
B
TX
RX
Sh
Sh
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A
B
A
B
TX
RX
Sh
Sh
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•
A
B
A
B
TX
RX
Sh
Sh
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Shield
Slave
Slave
Slave
Slave
MSS
(Master)