© Microhard Systems Inc.
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Appendix H: RS-485 Wiring
The n920 can be connected into a 2-wire or 4-wire RS-485 network. Transmission line termination
should be placed only at the extreme ends of the data line if the RS-485 network runs at high data
rates and has a long wiring run.
2-wire RS-485
Figure
J1
shows
a
typical
two-wire
configuration for an RS-485 connection to a
n920. Two wires are shared by transmitting and
receiving in a 2-wire configuration, so it is very
important for the modem to seize the line at the
right time when it transmits. Note again that a
transmission line termination is required if the
system has high data rates and long wiring
runs.
4-wire RS-485
A n920 can also be connected into a RS-485
network in a four-wire fashion as shown in
Figure
J2. In a four-wire network it is
necessary that one node be a master node and
all others be slaves. The network is connected
so that the master node communicates to all
slave nodes. All slave nodes communicate only
with the master node. Since the slave nodes
never listen to another slave response to the
master, a slave node cannot reply incorrectly to
another slave node.
Figure J1: 2-wire RS-485 Configuration
Figure J2: 4-wire RS-485 Configuration
Vin+ -
G
ND -
RxA -
RxB -
TxA -
TxB -
RS485/422
A (D-)
B (D+)
RxB (R+)
RxA (R-)
RS485/422
TxB -
TxA -
RxB -
RxA -
GND -
Vin+ -
TxA (D-)
TxB (D+)
