500 Ohm Shunt

This Conversion type applies only to a VoltageInputPoint used to read a 4-to-20mA sensor, where the UI input requires a 500 ohm resistor wired across (shunting) the input terminals. The input signal is 2 to 10V. As compared to a Linear or Generic Tabular conversion, this selection provides better resolution near the upper (20mA/10V) range of the input, compensating for input clamping protection the circuitry automatically applies when input voltage rises above 3.9V.

NOTE: Currently, selection of 500 Ohm Shunt produces a “secondary” selection for conversion type, with all conversion types shown again. However, only two secondary Conversion types are valid for 500 Ohm Shunt:

Linear if the 4-20mA sensor has a linear response (typical).
In the Scale and Offset fields for this Linear conversion, you enter whatever calculated values are needed to display in the appropriate units of measurement for that sensor. The following image shows an example configuration of such a point.

Figure 20. Example VoltageInputPoint used for 4-to-20mA sensor (Edit dialog from Point Manager)
For background on this example configuration, see “Scale and offset calculation example 2”.
Generic Tabular if the 4-20mA sensor is non-linear in response. This is much less typical.
In this case, you must enter (or import) a custom “source and result” .xml curve file, where the source (signal) values are between 0 to 10 (volts, where you multiply each mA value * 500), and the corresponding results are the measured values, in whatever units.

500 Ohm Shunt

Figure 20. Example VoltageInputPoint used for 4-to-20mA sensor (Edit dialog from Point Manager)

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