39
7
77
7 Appendix
Appendix Appendix
Appendix
Ⅱ
ⅡⅡ
Ⅱ
About RS485 B
About RS485 BAbout RS485 B
About RS485 Bus
usus
us
7.1
7.17.1
7.1 RS485 Bus Main Feature
RS485 Bus Main Feature RS485 Bus Main Feature
RS485 Bus Main Feature
RS485 is semi duplex communication cable of impedance 120Ω. Its max load amount is 32 effective loads
(including main control device and devices to be charged).
7.2
7.27.2
7.2 RS485
RS485RS485
RS485
Bus Transmission Distance
Bus Transmission DistanceBus Transmission Distance
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.
7.3
7.37.3
7.3 Connection
ConnectionConnection
Connection
Methods and Terminal R
Methods and Terminal RMethods and Terminal R
Methods and Terminal Resistance
esistanceesistance
esistance
1) RS485 bus requires all devices use the following connection methods. Both terminals shall have 120Ω
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 120Ω resistance information on the main board.
7.4
7.47.4
7.4 The Problem
The Problem The Problem
The Problem in
in in
in P
PP
Practical
ractical ractical
ractical U
UU
Use
sese
se
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
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