7. RS485 Modbus configuration
20
Detecting Gas Saving Lives
English
7.3 Cabling requirements
7.3.1 Calculating the minimum level of power required
The more
IRmax
detectors connected to the linear bus, the greater the power required
to run the system. To calculate the power required for a particular setup, it is necessary
to know the cable resistance between each pair of
IRmax
detectors. A current of at least
0.1mA must be allowed for each ‘hop’ between
IRmax
. The voltage to be applied can be
calculated by estimating the voltage drop across each ‘hop’ – at the end at least 12V must
remain to ensure that the last
IRmax
functions correctly.
Contact
Crowcon
for advice in specific cases. Alternatively, follow the steps outlined below
and the sample calculation shown in the next section.
1. The voltage must not fall below 12V, so start the calculation by setting the voltage at
the last
IRmax
in the line at that value.
2. Each
IRmax
may draw up to 0.1 A. Calculate the cable voltage loss of the first ‘hop’
between detectors by taking the ‘aggregate current’ to be 0.1A, and multiply this by the
cable resistance of the ‘hop’ between the last and the last but one
IRmax
.
3. Add this voltage drop to the initial 12V to get the lowest acceptable voltage at the last
but one
IRmax
. Add 0.1A to the value for the ‘aggregate current’ to get to 0.2mA, the
minimum current running through the last but one ‘hop’ of the bus. Multiply this by the
cable resistance for the last but one ‘hop’ to get the next voltage drop.
4. Repeat this process for each
IRmax
, accumulating the voltage losses that will occur
between each
IRmax
.
5. The maximum detector voltage of 30V must not be exceeded.
7.3.2 Sample calculation
As an example, here are the results of the calculation for six
IRmax
detectors spaced equally
250 m apart on cable with cross-sectional area of 1mm
2
. Each hop has a resistance of 4.6 ohms.
Station
Supply voltage
Supply current
1
12
0.1 A
2
12.46
0.2 A
3
13.38
0.3 A
4
14.76
0.4 A
5
16.6
0.5 A
6
18.9
0.6 A
Panel supply
21.66
0.6 A
