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HD-TVI Speed Dome User Manual
36
Appendix 2 RS485 Bus Connection
General Property of RS485 Bus
According to RS485 industry bus standard, RS485 is a half-duplex communication bus which has
120Ω characteristic impendence, the maximum load ability is 32 payloads (including controller
device and controlled device).
RS485 Bus Transmission Distance
When using 0.56mm (24AWG) twisted-pair line, according to different baud rate, the maximum
transmission distance theory table is shown as below:
Table A-1 Max. Distance of RS485 Transmission
Baud Rate
Max Distance
2400BPS
1800m
4800BPS
1200m
9600BPS
800m
The transmission distance will be decreased if we use the thinner cable, or use this product under
the strong electromagnetic interference situation, or there are lots of devices are added to the bus;
on the contrary, the transmission distance will be increased.
Connection Methods
RS485 industry bus standard require daisy-chain connection method between any devices, both
sides have to connect a 120Ω terminal resistance (show as Diagram 1), the simplified connection
method is shown as diagram 2, but the distance of “D” should not be too long.
1# 2# 3#
32#
…
120Ω
120Ω
Figure A-2 RS485 Connection 1
…
Controller
A+
B-
1#
2#
31#
A+ B-
D
Figure A-3 RS485 Connection 2
Problems in the Practical Application
Normally, users adopt star-shape connection method in construction, under this situation, the
terminal resistors must be connected between two farthest devices (as Figure 4, 1# and 15#), but
this connection method is not meets the requirement of the RS485 industry standard so that it will
lead to some problems such as signal reflection, anti-jamming ability decline when the devices are
faraway. At this time, the dome will be uncontrollable, or self-running, etc.
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