Appendix Ⅱ About Rs485 Bus; Rs485 Bus Main Feature; Rs485 Bus Transmission Distance; Connection Methods And Terminal Resistance - IC Realtime PTZ2300SIR User 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.

1) RS485 bus requires all devices use the following connection methods. Both terminals shall have 120Ω

resistance. See 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.

2) Please refer to chapter 2.1 for 120Ω resistance information on the main board.

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

Figure 7-1

Figure 7-2

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