ProVisual PTZ-501C – PTZ Keyboard Controller Manual
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P a g e
5 - Appendix
5.1 - RS485 Bus General Knowledge
●
RS485 Bus General Character
According to RS485 industrial standards, RS485 Bus is of half-duplexed data transmission cables with characteristic impedance as 120.
The maximum load capacity is 32 unit loads (including main controller and controller equipment)
●
Distance of RS485 Bus Transmission
While use the 0.56mm (24AWG) twisted cable as the communication, the farthest distance it can reach as follow based on the different
Baud rate
：
Baud rate
Max. distance
2400bps
1800M
4800bps
1200M
9600bps
800M
19200bps
600M
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 Terminal Resistance
The RS485 standards require a daisy-chain connection between the equipment. There must be termination resistance with 120
impedance at both ends of the connection (refer to picture 4-1.1).Please refer to picture 4-1.2 for simple connection But “D” should not
exceed 7m.
………..
1# 2# 3# 4# 32#
Figure 4-1.1
A+
D B-
A+ B-
…………
1# 2# 3# 4# 32#
Figure 4-1.2
●
Problems in practical use
In some circumstances user adopts a star configuration in practical connection. The termination resistors must be connected t o the
two equipments that are father away from each other, such as equipment1# and 15# (refer to picture 4-1.3). As the star configuration is not
in conformity with the
requirements of RS485 standards, problems such as signal reflections, lower anti-interference
performance arise
120
Ω
120
Ω
120
Ω
