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ALL-CAM2399-EP User Manual
© ALLNET GmbH Computersysteme 2015 – All rights reserved
Errors and omissions excepted
132
Appendix 6 RS485 Bus Connection
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).
RS485 Bus Transmission Distance
When using 0.56mm (24AWG) twisted-pair line, according to different baudrate, the
maximum transmission distance theory table is shown as below:
Max. Distance of RS485
Transmission
Baudrate
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.
Connection Methods
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.
Figure A-7 RS485 Connection 1
Figure A-8 RS485 Connection 2
Problems in the Practical Application
Normally, users adopt star-shape connection method in construction, under this