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Before You Begin
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Figure 18 Bus Wiring Configurations
For star configurations the cable length from
control unit to the most distant bus device should
be kept short, and should not exceed 100m.
There should be no more than four arms in the
star.
For a daisy chain configuration the total cable
length should not exceed 1,000m.
Bus Termination
The control unit bus uses the RS485 interface.
Because of this the ends of the line in some
configurations may be terminated to improve
performance in electrically noisy environments or
where there are long cable runs. The control unit.
expanders and keypads have a termination link on
their PCBs (see Figure 3 for the control unit and
Figures 8, 12 and 14 for the keypads). Fitting a
jumper to the pins adds a termination to the cable.
In a daisy chain configuration fit the termination
jumpers in the devices at each end of the chain
(see Fig 18 ).
In a star configuration:
If there are only two arms on the star then this
is the same as a daisy chain configuration
(see “Daisy Chain 1” in Figure 18). If required
fit the termination jumper at the devices on
the end of each arm.
If there are more than two arms AND two
cables are long while the remaining cables
are short (less than 10m) then it is possible to
terminate at the two devices on the ends of
the long cables.
If there are more than two arms BUT each
cable is more than 10m then
DO NOT
fit the
termination jumpers on any of the devices.
Voltage Drop
In order for the system to work correctly, the
voltage at each device must NOT drop below
10.5V even when running on the standby battery.
Eaton’s Security Business recommends that the
voltage at each device should stay above 12V.
Standard 7/0.2 alarm cable has a resistance of 8
Ohms per 100m per core. The voltage drop is
calculated using the following formula: V Drop =
Current drawn x cable length x 0.08 x 2.
Table 1 shows the voltage drop against the
current drawn and cable length.
The shaded area shows where the voltage drop
would cause the bus voltage to fall from 13.8V to
below 12.0V when using a single core.
