38
7.4
The Problem in Practical Use
There are 120
Ω
device terminal resistance in power socket. There are two connection ways.
The first one is shown as in Figure 7-3. It is the factory default connection method. Now the
jumper caps (connection board) are connected to the second and third socket, the 120
Ω
has not
connected.
1 2 3
J2
Default jumper setup
（
120
Ω
resistance has not connected
）
Figure 7-3
If you want to connect to the 120
Ω
resistance, you can remove the jumper caps from the second
and the third socket and then insert them to the first and second socket respectively. See Figure
7-4. Now you has connected to the 120
Ω
resistance.
1 2 3
J2
Jumper setup
（
Connect to 120
Ω
resistance
）
Figure 7-4
7.5
The Problem in Practical Use
In practical usage, we usually adopt star type connection. The terminal resistance shall connect
to the furthest two devices (Such as device 1# and device 15# in Figure 7-5 ). But this connection
way does not conform to RS485 Bus standard. When the distances between devices are too
long, the signal reflection occurs and anti-jamming decreases, thus the signal reliability becomes
very low. You can see speed dome is not under control or speed dome is running automatically
and can not stop.
Figure 7-5
