Vista powerdome pro series, Руководство по установке

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PowerDome PRO manual V3-6
73
Appendix II: RS-485 Bus Basic Knowledge
1. Characteristics of RS485 Bus
As specified by RS485 standards, RS485 Bus is of half-duplex data transmission cables with characteristic
impedance as 120
Ω
. 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
Transmission Distance (Max.)
2400bps 5,906ft (1800m)
4800bps 3,937ft (1200m)
9600bps 2,625ft (800m)
If the user selects thinner cables, or installs the dome in an environment with strong electromagnetic
interference, or connects a large numbers of equipment to the RS485 Bus, the maximum transmitting
distance will decrease. 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 120
Ω
impedance at both ends of the connection (refer to following
pictures).
Please refer to picture below for a simple connection.
Distance ―D‖ should not exceed 7 meters.
The 120
Ω
comes fitted in the un-terminated position on the
jumper on pin 2 & pin 3.
To terminate the dome, pull the plug from the jumper and move it
to pin 1 & pin 2.
D
Reverse of telemetry
board