18
Figure 4
‑
7 Schema of transmission distance
Type of networking transmission line
The commonly-used connecting lines consist
①
shielded twisted pair,
②
ordinary
twisted-pair,
③
shielded line,
④
plain conductor and otherwise. By our test, the effective
transmission distance of
①
and
②
under a transmission rate of 100K reaches about
1500m
~
2000m and 3000m
~
4000m under a transmission rate of 20K, while the transmission
distance of
③
and
④
is only 1/5
~
1/3 that of
①
and
②
.Therefore, it is suggested to use the
ordinary twisted-pair as the networking line and to use the shielded twisted pair for the
applications with more external disturbances (for example, many power cables available inside
the cable trough or with high-power radio transmitters nearby).
To sum up, the transmission rate shall be selected properly based on networking distance
and host number of the engineering. Then select proper transmission line according to the
transmission rate. In normal applications, generally use the twisted pair to connect 2-5 hosts,
with a transmission distance less than 1000m and a transmission rate of 20K.
The number of transmit-receive packages on the bus may
be observed by the menu of “special operation” – “bus quality inspection”. If both
numbers differ greatly, it means the success rate of communications is relatively low,
and the transmission rate shall be reduced or the transmission environment shall be
improved.
The networking of 20 hosts is achievable on the system. The abnormity of any
host in the network will not cause fault of the overall system. All hosts share all
alarm messages (including fire alarm, request, feedback and fault) over the
network.
As the concentrator in the network, host 1 will not only provide a time
reference, it will also support all network command operations. Therefore, a host
must be set as the concentrator in the network.
The hosts involved in the network must be registered and each host must be
aware of the online hosts over the network.
The connecting line of networked host must be of serial
connection; star connection or ring connection is prohibited! Otherwise, the
communication will become abnormal!
Figure 4
‑
8 Connection schematic diagram of host networking