9. 4 RS-485 Bus basic knowledge
The RS485Bus standards require a daisy-chain
connection between the equipments. There must be
termination resistors with 120
(as the picture 9-
Please refer to picture 9-4.2 for simple
D should not exceed 7m.
Ø
Connection and termination resistor
4.1).
connection.
120
1#
2#
3#
4#
32#
Picture 9-4.1
120
120
1#
2#
3#
4#
32#
D
A+
B-
A+
B-
Picture 9-4.2
120
2400BPS
4800BPS
9600BPS
1800m
1200m
800m
Baud rate
Max distance
19200BPS
600m
Ø
The connection of 120
temination resistor:
The termination resistor is ready on the protocol
PCB. The are two kinds of connection(as show 9-4.3
form). It is the factory default connection. The jumper
cap of switchboard is seated on pin 2 & Pin 3 and the
termination resistor120
is not connected.
When connecting the 120
termination resistor,
user should pull out the protocol PCB and plug the
jumper on Pin 1&pin 2. Install the PCB back and the
termination resistor is connected.
(as show the picture 9-4.3)
Factory default setting
Connection of120 setting
1
2
3
On
On
Off
On means insert the jumper cap
Off
On
On
Picture 9-4.3
As specified by RS485 standard, RS485 Bus is
of half-duplexed data transmission cables with
characteristic impedance as 120
. The maximum
load capacity is 32 unit loads(including main
controller and controller equipment)
Ø
Characteristics of RS485 Bus
Ø
Transmission distances of RS485 Bus
When user selects the 0.56mm(24AWG) twisted
pair wires as data transmission cable, the maximum
theoretical transmitting distance is as below:
If user selects thinner cables, or installs the
dome in an environment with strong electromagnetic
interference, or connects lots of equipment to the
RS485Bus, the maximum transmitting distance will
be decreased. To increase the maximum transmitting
distance, do the contrary.
35
Appendix