SPEED DOME CAMERA
APPENDIX III: RS485 BUS
BASIC KNOWLEDGE
1
Basic Property
RS485 Bus is specified by RS485 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 con-
trolled 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 elec-
tromagnetic interference, or connects large
number of equipment to the RS485 Bus, the
maximum transmitting distance will be
decreased.
3
Connection and termination resistor
3-1 The RS485 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
ON
O FF
4
Best Practice
In some circumstances user adopts a star
configuration in practical connection. The
termination resistors must be connected to
the two equipment that are farthest away
from each other, such as equipment 1# and
15# in the following picture.
As the star configuration is not in conformity
with the requirements of RS485 standards,
problems such as signal reflections, lower
anti-interference performance arise when the
cables are long in the connection.
The reliability of control signals is decreased
with the phenomena that the dome does not
respond to or just responds at intervals to
the controller, or does continuous operation
without stop ( refer to following picture).
5
RS485
Trouble
Possible Cause
Solution
Dome can do
self-testing
but cannot
be controlled.
1. The address and baud
rate settings of dome are
not in conformity with
those of controller.
2. The + and -
connection of RS485 Bus
is
incorrect.
3. Wiring is not fully
seated.
4. There is breakage in
the middle section of the
RS485 Bus
1. Change the address
and baud rate of
controller or dome.
2. Correct the
connection.
3. Make sure the
connections are fully
seated.
