Technical data

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Appendix Ⅲ: RS485 Bus Basic Knowledge
1. Characteristics of RS485 Bus
As specified by RS485 standards, RS485 Bus is of half-duplexed
data transmission cables with characteristic impedance as 120Ω.
The maximum load capacity is 32 unit loads (including main
controller and controlled equipment).
2. Transmission distances of RS485 Bus
When user selects the 0.56mm (24AWG) twisted pair wires as data
transmission cable, the maximum theoretical transmitting distances
are as follows:
Baud Rate
Maximum Transmitting
Distance
2400 Bps 1800m
4800 Bps 1200m
9600Bps 800m
Table 7
If user selects thinner cables, or installs the dome in an environment
with strong electromagnetic interference, or connects lots of
equipment to the RS485 Bus, the maximum transmitting distance
will be decreased. To increase the maximum transmitting distance,
do the contrary.
3. Connection and termination resistor
The RS485 standards require a daisy-chain connection between
the equipment. There must be termination resistors with 120Ω
impedance at both ends of the connection (refer to Picture 32).
Please refer to Picture 33 for simple connection. “D”should not
exceed 7m.
120Ω 120Ω
1# 2# 3# 4# 32#
. . . . .
Figure 32