Lantronix MSS1-T, Reference Manual

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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.
• The MSS can be used in either two-wire or four-wire mode. Refer to the fol-
lowing sections to determine which mode to use.
• 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
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.
•
•
•
•
TXA
TXB
TX
RX
Shield
A
A
B
B
A
B
B
A
RX
TX
Shield
Shield
Shield
•
•
•
A
B
A
B
TX
RX
Sh
Sh
•
•
•
•
•
•
A
B
A
B
TX
RX
Sh
Sh
•
•
•
•
•
•
A
B
A
B
TX
RX
Sh
Sh
•
•
•
• •
Shield
Slave
Slave
Slave
Slave
MSS
(Master)