Last Revised 05/09/2013
11
This next diagram is a slight variation on the Daisy Chain arrangement. Again it’s one cable
going out to all the domes but instead of the cable going into each dome then back out to the
next one, a junction box is used to “Spur-Off” to each dome. Whilst this can be done THE
SPUR LENGTH (D) must be NO MORE THAN 10 metres!!!!!!!!!!!! The overall cable
length between the Controller and the last dome is limited to around 300 metres in such an
arrangement.
120
1#
2#
3#
31#
120
D
A+
B-
A+
B-
. . . . .
. . . . .
Main controller
Daisy-Chain connection WITH SHORT SPURS for the RS485 PTZ control
signal
(one main radial with very short spurs to each dome off it, keeping the spurs to less than 10 meters)
STAR method of connection.
In some circumstances you may need to adopt a star configuration for practical purposes. For
instance, all the domes may be so scattered on a large site that running out separate spurs to
each dome in a “STAR” array is the only practical solution.
So how do you do this in practice?
The termination resistors must be connected to the two domes that are farthest away from
each other, such as domes 3 and 5 in the following “Star diagram”. Note that all the other
domes do not have the 120ohm resistor connected. To fit them, connect them across the two
connections in the PTZ camera terminal connector.
As the star configuration is not in conformity with the requirements of RS485 standards,
problems such as signal reflections may arise, especially when there are long cable
connections. The results are that control signals are decreased and the dome may not respond
to, or just responds intermittently to the
controller.
If your STAR circuit is not too extensive
with each spur in the region of 20-50 metres
you can expect quite good reliable
performance using this technique. If you
experience any problems though, there is a
RS485 distribution box available CODE
PT750 to help overcome any problems.
The Star circuit for wiring PTZ’s.
The two furthest domes need the
120ohm resistors enabling, by moving
the “Jumper”
In this example it’s domes 3 &5.
The Star Diagram
