Appendix
A+
120Ω
B-
120Ω
120Ω
5.1 RS485 bus line basics
Characteristics of RS485 bus line
As specified by RS485 standards, RS485 bus line is a
half-duplex data transmission cable with characteristic
impedance of 120Ω. The maximum load capacity is 32
unit loads (including the main device and the controlled
devices).
Transmission distance of RS485 bus line
When the user selects the 0.56mm (24AWG) twisted
pair wires, theoretically the maximum transmission
distances are as the table shows below.
If user selects a thinner cable, or installs the dome in
an environment with stronger electromagnetic interference,
or connects lots of devices to the RS485 bus line, the
maximum transmission distance will decrease. To increase
the maximum transmission distance, the user should do the
contrary.
Problems in practice
In practice, the user usually adopts a star
configuration, and in this case, terminal resistors must
locate at the two devices that are farthest away (see
Picture 5-1.3 device1# and 15#). However, the star
configuration is not in conformity with the RS485
standards, so problems such as signal reflections, poor
anti-interference performance will arise when the
devices are relatively far away from each other . The
reliability of the control signal will decrease: the
dome dose not responds to or just responds at intervals
to the controller, or the dome continuously repeats an
operation.
Under such circumstances, we recommend the usage
of RS485 distributor. The distributor can change the star
configuration to a connection configuration that is
stipulated in the RS485 standards, and this configuration
can achieve reliable data transmission (see Picture 5-1.4)
1
20Ω
1#
Connection and terminal resistance
The RS485 standards require a daisy-chain connection
between each device. There must be terminal resistors of 120Ω
at both ends of the connection (see Picture 5-1.1).
Please refer to picture 5-1.2 for simple connection. The
length of the “D” area should not exceed 7m.
32#
120Ω
1 5#
6#
Main Controllor
Picture 5-1.3
12
0Ω
120Ω
RS485 Distributor
1 #
2 #
3 #
4#
32#
Picture 5-1.1
1#
2#
A+
D
B-
120Ω
3#
A+ B-
120Ω
1 #
2#
3 #
4 #
32#
Picture 5-1.2
Picture 5-1.4
Baud Rate
2400bps
4800bps
9600bps
19200bps
Maximum Distance
1800m
1200m
800m
600m
