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APPENDIX IV
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RS-485 BUS
BASIC
KNOWLEDGE
1. Basic Property
RS-485 Bus is specified by RS-485 standards.
It is of half-duplex data transmission cables
with characteristic impedance as 120.
The maximum load capacity is 32 effective
loads (including main controller and controlled
equipment).
2. Transmission Distance
For 0.56mm (24AWG) twisted pair wires as
data transmission cable, the maximum
theoretical transmitting distances are as
follows
Baud Rate
Max. Transmit Distance
2400BPS
1200m
4800BPS
1000m
9600BPS
800m
In case of thinner cables, or installs the dome in
an environment with strong electromagnetic
interference, or connects large number of
equipment to the RS-485 Bus, the maximum
transmitting distance will be decreased.
3. Connection and termination resistor
3-1 The RS-485 standards require daisy-chain
connection between the equipment. There must
be termination resistors with 120 ohm
impedance at both ends of the connection
(Refer to following pictures).
Please refer to following picture for simple
connection. “D” should not exceed 7m.
2 3
1 2
O N
O F F