32
7
Appendix Ⅱ About RS485 Bus
7.1
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).
7.2
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:
z
The communication cable is a little bit thin;
z
The surrounding environment has strong electromagnetic interference;
z
There are too much devices connected to the RS485 bus;
And vice versa, the max transmission distance shall become longer.
7.3
Connection Methods and Terminal Resistance
1) RS485 bus requires all devices use the following connection methods. Both terminals shall have 120
Ω
resistance. See Figure 7-1.
Figure 7-1
Or you can refer to for the simplified way as in Figure 7-2. But the distance between “D” shall not exceed 7M.
Figure 7-2
2) Please refer to chapter 2.1 for 120
Ω
resistance information on the main board.
7.4
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 7-3 ). But this connection way does not
Baud Rate
Max Distance
2400 BPS
1800M
4800 BPS
1200M
9600 BPS
800M
