Planning the Installation
Page 17
Bus termination
In some cases, the ends of the bus may need to be terminated to improve performance in
electrically noisy environments or where there are long cable runs. The control unit and
bus devices have a termination link on their PCB. Fitting a jumper to the link adds a
termination to the cable.
In a daisy-chain configuration, fit the termination jumpers in the devices at each end of the
chain. In a star configuration, terminate at the two devices on the ends of the longest
cables (Figure 4).
Figure 4. Bus Termination
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. Preferably, the voltage at each device
should stay above 12V to avoid unexpected behaviour.
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.
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.
