500 Ohm Shunt

(Starting in AX-3.4 and in builds 3.3.28, 3.2.23, and 3.1.33) 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. If this conversion type is not available, select Linear conversion instead.

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.

Currently, selection of 500 Ohm Shunt produces a “secondary” selection for conversion type, with all conversion types shown again. Typically, only the Linear type is valid, providing the 4-20mA sensor has a linear response.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. Figure 12 shows an example configuration of such a point.

Figure 12. Example VoltageInputPoint used for 4-to-20mA input (Edit dialog from Point Manager)

For background on this example configuration, see example 2.

In the unusual case of a non-linear 4-20mA sensor, providing you are using AX-3.4 or later ndio and have the kitIo module installed, the secondary conversion type “Generic Tabular” can be used. For more details, see

Default

(The default selection). Conversion between “similar units” is automatically performed within the proxy point. For example, for an ThermistorInputPoint if you set the units in its facets to degrees F, the point output automatically converts to the appropriate value, from degrees C.

If you set the parent point’s Facets to dissimilar units (often the case with a VoltageInputPoint), the parent point has a fault status to indicate a configuration error. In this case, you must select Linear conversion, and also set “Units” in the point’s “LinearCalibrationExt” to match the point’s facets. See Scale and offset calculation (linear) and Linear Calibration Ext.

Generic Tabular

(AX-3.4 and later only, requires kitIo module to be installed). This Conversion type allows non-linear support for devices other than for thermistor temperature sensors with units in temperature. Generic Tabular uses a “lookup table” method similar to the “Thermistor Tabular” conversion, but without predefined output units. Selection provides a popup Tabular Conversion Dialog in which you can enter a custom “source-to-result” non-linear curve, including the ability to import and export response curve files. For more details, see Non-linear sensor support and Curve File Import/Export notes.

Linear

Applies to VoltageInputPoint, ResistiveInputPoint, and VoltageOutputWritable points. For these points, you typically want the point’s output value in some units other than Device Facets (voltage or resistance). The Linear selection provides two fields to make the transition:

For related details, see Scale and offset calculation (linear).

For a VoltageInputPoint used for a 4-to-20mA sensor (shunt resistor on UI input), another primary conversion type should be used, if available: 500 Ohm Shunt, with Linear typically selected as the “secondary” conversion. See more about that other conversion type above.

Reverse Polarity

Useful in a BooleanInputPoint point or BooleanOutputWritable to reverse the logic of the hardware binary input or output.

Be careful in the use of the reverse polarity conversion, as it may lead to later confusion when troubleshooting logic or communications problems.

Thermistor Type 3

Applies to a ThermistorInputPoint point, where this selection provides a “built-in” input resistance-to-temperature value response curve for Type 3 Thermistor temperature sensors.

Tabular Thermistor

Applies to a ThermistorInputPoint point, where this selection provides a control for a popup dialog for a custom resistance-to-temperature value response curve, including ability to import and export curve response files. For more details, see Tabular Thermistor and Curve File Import/Export notes.