Appendix Ⅱ About Rs485 Bus; Rs485 Bus Main Feature; Rs485 Bus Transmission Distance; The Problem In Practical Use - Optiview OV23X Installation Manual

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7 APPENDIX Ⅱ ABOUT RS485 BUS

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).

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.

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 7-1 ). 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.

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 7-2.

Baud Rate

Max Distance

2400 BPS

1800M

4800 BPS

1200M

9600 BPS

800M

Figure 7-1

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