5 Inch Network High-definition Speed Dome User Manual
47
Appendix 3 RS485 Bus Connection
1. General Property of RS485 Bus
According to RS485 industry bus standard, RS485 is a half-duplex communication bus which has 120Ω characteristic impendence, the
maximum load ability is 32 payloads (including controller device and controlled device).
2. RS485 Bus Transmission Distance
When using 0.56mm (24AWG) twisted-pair line, according to different baud rate, the max transmission distance theory table is shown
as below:
Baud Rate Max Distance
2400BPS 1800m
4800BPS 1200m
9600BPS 800m
The transmission distance will be decreased if we use the thinner cable, or use this product under the strong electromagnetic
interference situation, or there are lots of devices are added to the bus; on the contrary, the transmission distance will be increased.
3. Connection Method and Terminal Resistance
1) RS485 industry bus standard require daisy-chain connection method between any devices, both sides have to connect a 120Ω
terminal resistance (show as Diagram 1), the simplified connection method is shown as diagram 2, but the distance of “D”
should not be too long.
Diagram 1
Diagram 2
2) Connection of 120Ω terminal resistor
The 120Ω terminal resistor can be connected through the DIP switch on the communications board, as shown in Figure3. For a new
dome, the 120Ω matching resistor is defaulted as unconnected, switch on the eighth bit of SW2, it will be connected. Conversely,
switch off the eighth bit of SW2, it will be unconnected.
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