10
If your STAR circuit is not too extensive
with each spur in the region of 20-50 meters
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.
Overcoming RS485 data loss using an RS485 distributor
In the real world not everything always works exactly as it’
s expected to!
RS485 data signals that control the dome’
s movements are tiny signals that can get corrupted for many
reasons. Poor cable quality, not using a PAIR of cores from a CAT5 but using one core from TWO separate
pairs, running the CAT5 cable near mains equipment such as florescent lighting all will have a detrimental
effect on the signal. These are things that you can correct with good installation practices.
Where you wish to run several separate CAT5 cables out to send the RS485 data signal out to the domes you
are in effect correcting the STAR method of RS485 data distribution. As previously mentioned the problem
with the Star method is that it is not actually designed for RS485 but generally works okay if you follow the
previous notes on getting the 120ohm resistor setting right, as per the previous notes.
One way that takes the guesswork out of installing the Star method is to utilise an RS485 distributor. This has
the advantage that the RS485 signal is correctly distributed to the domes so that they behave as expected.
You can create up to 4 spurs to the domes and put up to 4 domes on each spur. Just like the Daisy chain
method the end dome on each spur needs to have the 120ohm resistor enabled.
Although the RS485 distributor is a small additional expense, it takes some of the guess work out of the
installation design and gives a more flexible approach to cabling which itself can save time and money on
the installation. Not forgetting you get more predictable results!
The RS485 distributor (PTZ750) amplifies the RS485 control signal and distributes it evenly to 4 separate
spurs, each spur can have up to 4 domes. This means that you could theoretically have up to 4 individual
spurs of over 1000mtrs each to control up to 16 PTZ domes in total.
The Star circuit for wiring PTZ’
s.
The two furthest domes need the 120ohm
resistors enabling, by moving the dip switch.
In this example it’
s domes 3 &5.
The Star Diagram