Characteristics of RS485 Bus
As specified by RS485 standards, RS485 Bus is of half-duplexed data
transmission cables with characteristic impedance as 120
Ω
. The maximum
load capacity is 32 unit loads (including main controller and controlled
equipment).
Transmission distances of RS485 Bus
When user selects the 0.56mm (24AWG) twisted pair wires as data trans-
mission cable, the maximum theoretical transmitting distances are as follows:
If user selects thinner cables, or installs the dome in an environment with
strong electromagnetic interference, or connects lots of equipment to the
RS485 Bus, the maximum transmitting distance will be decreased. To
increase the maximum transmitting distance, do the contrary.
Connection and termination resistor
The RS485 standards require a daisy-chain connection between the
equipment. There must be termination resistors with 120
Ω
impedance at
both ends of the connection (refer to Figure 3).
Please refer to Figure 4 for simple connection. "D" should not exceed 7m.
1
2
3
Baud Rate
2400 Bps
4800 Bps
9600Bps
Maximum Transmitting Distance
1800m
1200m
800m
Appendix
34
VI. Appendix
Appendix I:
RS485 Bus Basic Knowledge
1#
2#
3#
31#
120
Ω
120
Ω
A+
B-
A+
B-
D
Main controller
1#
2#
3#
4#
32#
120
Ω
120
Ω
Figure 3
Figure 4
