Rs-485 Networking Overview; Rs-485 Protocols; Two-Wire Mode - Lantronix MSS485 Installation Manual

RS-485 Networking Overview

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 following 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
Shield
A
TX
B
•
B
RX
A
Shield
MSS
(Master)
1 - 2
Introduction
• •
Shield
• TXA •
•
TXB
Sh
•
A
•
TX TX
B
•
B
RX RX
A
Sh
Slave
Slave
• •
• •
• •
Sh Sh
• •
A A
• •
TX
B B
• •
B B
RX
A A
Sh Sh
Slave
Shield
•
A
•
RX
B
•
B
TX
A
Shield
Slave