Appendix B RS485Bus Common Sense
1. RS485 Bus Basic Features
According to RS485 Industry Bus Standard, RS485 Industry Bus is semiduplex communication bus with
120
Ω
Resistance, the maximum loading capability is 32 effective loads (including main control equipment and
controlled plant)
2. RS485 Bus Transmission Length
When using 0.56mm (24AWG) cross wire as communication wire, the maximum transmission length in theory
is relevant with the baud rate, the details are showed in the following tables:
Baud Rate
The maximum transmission length
2400bps 1800m
4800bps 1200m
9600bps 800m
When using thin communication wire, or using the products in strong electromagnetic interference environment,
or many devices are connected on the bus, the maximum length should become shorter accordingly, on the
contrary, the maximum length should become longer.
3. Connection Way and Terminal Resistance
1)
RS485 industry bus standards ask the devices being connected in daisy chained way, there are 120
Ω
terminal resistance at the two ends(show as figure one); simplified connection can use the connection
method shown in figure two, but the length of “D” should be smaller than 7 meters.
Figure 1
Figure 2
2)
The Connection Method of Speed dome Terminal 120
Ω
Resistance
Speed dome Terminal 120
Ω
matched resistance can be connected by setting the dial-up switches on the
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