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Appendix
: RS485 Bus Basic Knowledge
Ⅰ
1. Characteristics of RS485 Bus
As specified by RS485 standards, RS485 Bus is of half-duplexed data transmission cables with
characteristic impedance as 120Ω. The maximum load capacity is 32 unit loads (including main
controller and controlled equipment).
2. Transmission distances of RS485 Bus
When user selects the 0.56mm (24AWG) twisted pair wires as data transmission cable, the maximum
theoretical transmitting distances 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 RS485 Bus, the maximum transmitting distance will
be decreased. To increase the maximum transmitting distance, do the contrary.
3. Connection and termination resistor
The RS485 standards require a daisy-chain connection between the equipment. There must be
termination resistors with 120Ωimpedance at both ends of the connection (refer to Picture 3).
Please refer to Picture 4 for simple connection. “D” should not exceed 7m.
120
Ω
120
Ω
1#
2#
3#
4#
3
2#
. . . . .
Figure 3
1
20
Ω
1#
2#
3#
31#
120
Ω
D
A
+
B
-
A+
B-
.....
.....
Mai
n
cont
r
ol
l
er
Figure 4
The connection of 120Ω termination resistor: The termination resistor is ready on the Protocol PCB.
There are two kinds of connection. Refer to the Protocol PCB jumper setting form (refer to Picture 2).
1) In the Picture it is the factory default connection. The jumper is seated on Pin2&Pin3 and the
termination resistor is not connected.
2) when connecting the 120Ω termination resistor, user should plug the jumper on Pin1&Pin2. and the
termination resistor is connected.
4. Problems in practical connection
In some circumstances user adopts a star configuration in practical connection. The termination
resistors must be connected to the two equipment that are farthest away from each other, such as equipment
1# and 15# in Picture 44. As the star configuration is not in conformity with the requirements of RS485
standards, problems such as signal reflections, lower anti-interference performance arise when the cables
