© Copyright Qvis® All documentation rights reserved.
30
10
APPENDIX ABOUT RS485 BUS
10.1
RS485 Bus Main Feature
RS485 is semi duplex communication cable with an impedance of 120
Ω
. Its maximum load amount is 32
effective loads (including main control device and the devices that need to be charged).
10.2
RS485 Bus Transmission Distance
When we take a 0.56mm (24AWG) twisted-pair to use as a communication cable, the maximum transmission
distances (theoretically) are listed below (according to different baud rates).
In the following situations, the maximum transmission distance will become shorter according to these three
factors:
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;
The opposite of these factors will mean the maximum transmission distance will become longer.
10.3
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 10-1 ). But this connection method does not conform
to the RS485 Bus standard. When the distances between devices are too long, signal reflection occurs and anti-
jamming decreases, thus the signal reliability becomes very low. You can see that the speed dome is not under
control or the speed dome is running automatically and cannot stop.
Figure 10-1
Baud Rate Max Distance
2400 BPS
1800M
4800 BPS
1200M
9600 BPS
800M
1#
32#
6#
15#
