78
English
Z
Appendix
Ⅲ：
RS485 bus common sense
1
、
RS485 bus basic features
According to RS485 industry standard, it belongs to special impedance 120
Ω
semiduplex
communication bus, the max load capacity is 32 effective loads
（
contain master equipments and
slave equipments
）
2
、
RS485 bus transmission distance
When we use 0.56mm(24AWG)twisted-pair as communication cables,according to the different
Baud Rate
，
the max transmission distance’s theoretical value as follows
：
If the communication cable is quite thin, or it is used in the area of strong electromagnetism
disturbance, or too many equipments are connected to the RS485,then the max transmission distance
will be shortened; Otherwise, it will be lengthened
3
、
Connecting pattern and terminal resistance
3-1
RS485industry standard requires that various equipments should be complied with
chrysanthemum-style connecting methods and both ends have to own 120
Ω
terminal resistance
（
As FIG 57
）
or for a simple one as FIG 58
，
but distance of“D”segment shouldn’t exceed
7meter.
z
Controlling Equipments
：
refer to matrix, keyboardand etc.
z
1#-32#
：
stand for dome structure
3-2 Equipment terminal resistance120
Ω
connecting methods as shown on FIG59
：
Equipments’terminal resistance on the alarming communication board has been existed: The
eighth one of SW3 on the alarming communication board is set to be “ON”by the connecting
method. (refer to appendix
Ⅳ
SW3 DIP Switch Setting)
。
Baud Rate
Max Distance
2400BPS 1800m
4800BPS 1200m
9600BPS 800m
