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4.5 RS-485 Network and Impedance
Transmission distances of RS-485 Bus
The 0.56mm (24AWG) twisted pair or higher grade wires are recommended for data
transmission cable.
The maximum theoretical transmitting distances, for the AWG#24, are as follows:
Baud Rate Maximum Transmitting Distance
2400 Bps 1800m
4800 Bps 1200m
9600Bps 800m
If user selects thinner cables, or installs the dome in an environment with strong
electromagnetic interference, or connects lots of equipment to the RS-485 bus, the
maximum transmitting distance will be decreased. To increase the maximum
transmitting distance, do the contrary, i.e. use thicker wire and keep the cable away
from the interference.
Connection and termination resistor
The RS-485 standard requires a daisy-chain connection among the equipments.
There must be termination resistors of proper impedance (typically 120 ohms,
within the range from 90 to 250 ohms) at both ends of the connection.
120Ω 120Ω
1# 2# 3# 4# 32#
. . . . .
120Ω
1# 2# 3# 31#
120Ω
D
A+
B-
A+ B -
.....
.....
Main controller
. .
User’s manual 13