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5
APPENDIX ABOUT RS485 BUS
5.1
RS485 Bus Main Feature
RS485 is semi duplex communication cable of impedance 120
Ω
. Its max load amount is 32
effective loads (including main control device and devices to be charged).
5.2
RS485 Bus Transmission Distance
When we take 0.56mm (24AWG) twisted-pair as communication cable, the max transmission
distance (theoretically) are listed below (according to different baud rates).
In the following situations, the max transmission distance shall become shorter accordingly:
The communication cable is a little bit thin;
The surrounding environment has strong electromagnetic interference;
There are too much devices connected to the RS485 bus;
And vice versa, the max transmission distance shall become longer.
5.3
The Problem in Practical Use
In practical usage, we usually adopt star type connection. The terminal resistance shall connect
to the furthest two devices (Such as device 1# and device 15# in Figure 5-1 ). But this connection
way does not conform to RS485 Bus standard. When the distances between devices are too
long, the signal reflection occurs and anti-jamming decreases, thus the signal reliability becomes
very low. You can see speed dome is not under control or speed dome is running automatically
and can not stop.
Figure 5-1
In this situation, we recommend RS485 distributor. This device can turn star type connection into
the connection that conforms to RS485 bus industry standard, which can avoid the above
mentioned problems and enhance communication reliability. See Figure 5-2.
Baud Rate Max Distance
2400 BPS
1800M
4800 BPS
1200M
9600 BPS
800M
