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7. CONNECTION OF RS485 BUS AND TERMINATION RESISTOR
(1) Characteristics of RS485 Bus
As specified by RS485 standards. RS485 Bus is of half duplexed data transmission cables with characteristic
impedance as 12. The maximum load is 32 unit loads (including main controller and controlled equipment.)
(2) The RS485standarda require a daisy-chain connection between the equipment. There must be termination
resistor with 120 ohms impedance at both ends of the connection (refer to the following FIGURE)
When No. 10 bit of the Dip is set to “ON”, the 120 ohms termination resistor is connected.
(3) Problem in Practical Connection
In some circumstances user adopts a star configuration in practical connection. The termination resistors must be
connected to the two equipments (No. 6 and No. 10) that are farthest away from each other. But the connection
does not meet the RS485 standards.
When the cable distance of equipments are far away, some problems, such as signal reflection, anti-jamming
ability decrease are easily occur and result in the reliability decline of control signal. The resulted phenomena
represent that the camera is out of control completely or interruptedly or operates automatically and fails to
stop, etc. In such circumstances the factory recommends the RS485 Signal Distributor. The distributor can
change the star configuration connection to the mode of connection stipulated in the RS485 standards. The
mew connection achieves reliable data transmission.
RS485 Distributor
Control
120
Ω
120
Ω
120
Ω
1#
6#
12#
RS485 Distributor
