Cabling Requirements; Calculating The Minimum Level Of Power Required; Sample Calculation - Crowcon IRmax Instructions D'installation, D'utilisation Et De Maintenance

7. RS485 Modbus configuration

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
Station Supply voltage
1 12
2 12.46
3 13.38
4 14.76
5 16.6
6 18.9
Panel supply 21.66
2 . Each hop has a resistance of 4.6 ohms.
Supply current
0.1 A
0.2 A
0.3 A
0.4 A
0.5 A
0.6 A
0.6 A
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