Appendix A Rs485 Bus Basic Knowledge - Hitachi VK-S454ER User Manual

Appendix A RS485 Bus Basic Knowledge

Characteristics of RS485 Bus
1.
As specified by RS485 standards, RS485 Bus is of semiduplex data transmission cables with characteristic impedance as 120 .
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:
3. Connection and terminal resistor
a) The RS485 standards require a daisy-chain connection between the equipment. There must be terminal resistors with
120 impedance at both ends of the connection (refer to Figure 16). Please refer to Figure 17 simple connection. "D" should
not exceed 7m.
b) The connection of 120 terminal resistor: The terminal resistor is ready on the Protocol PCB. There are two kinds of
connection. Refer to the Protocol PCB jumper setting form (refer to Figure 18). In the Figure 16 it is the factory default
connection. The jumper is seated on Pin2&Pin3 and the terminal resistor is not connected. When connecting the 120
terminal resistor, user should plug the jumper on Pin1&Pin2, and the terminal resistor is connected.
4. Problems in practical connection
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 Figure19. 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 Figure20).
Band Rate
2400 Bps
4800 Bps
9600Bps
120
1# 2#
Maximum Transmitting Distance
1800m
1200m
800m
Table 4
3# 4#
Figure 17
Figure 18
High Speed Dome Camera User's Manual
. . . . .
120
32#
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