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1.3.2 RS485 Networks
RS485 permits a balanced transmission line to be shared in a party line or multi-drop configuration. As
many as 32 driver / receiver pairs can share a multi-drop network on a single two wire bus. The length of
the network is limited to 4,000 ft. between the first node and the last node. RS485 can be used in two-wire
or four-wire multi-drop network applications.
Figure 3 shows a typical RS485 two-wire multi-drop network. The tri-state capabilities of 485 allow a single
pair of wires to share transmit and receive signals for half-duplex communications. In this configuration, it
is important to prevent more than one device from transmitting at the same time. This is controlled by
software and the communications protocol. Note that the transmission line is terminated on both ends of the
line but not at drop points in the middle of the line. Termination is only required with high data rates and / or
long wire runs.
Note: Do NOT install termination resistors on short wire networks. See the Application Notes on the product CD for more
information about networks and termination procedures.
D
R
DE
DI
RO
D
R
DE
DI
RO
CP1
TX
RX
TXDA (Pin 7)
RXDB (Pin 4)
SGND (Pin 3)
CP1
DE
CP1
D
R
DI
RO
TX
TXDA
RXDB
SGND
D
R
DE
DI
RO
RX
CP1
DE
CP1
D
R
DI
RO
TX
TXDA
RXDB
SGND
D
R
DE
DI
RO
RX
CP1
RXDA (Pin 5)
TXDB (Pin 6)
Figure 3 - RS485 Network
NET485 User Guide 1-5