Custom scale/offset - Johnson Controls - LIT-1201668 - System Integration - LonWorks Integration - 12.0

LonWorks Network Integration with Network Engines and LCS Technical Bulletin

Johnson Controls
Product name
LonWorks Integration
Document type
Technical Bulletin
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LonWorks network communications are broadcast in metric units. The LonWorks Integration can automatically convert those units for most cases. In those situations where automatic conversion is insufficient, Custom Scale/Offset provides the ability to scale analog values through the Custom Scale/Offset field on the Hardware tab of analog objects.

For the normal case, a temperature value polled from an NV on a LonWorks device using a SNVT_temp_p has a metric value in Deg C. By setting the Hardware tab (field) units to Deg C and the Focus tab (display) units to Deg F, the conversion between the two is performed automatically.

This conversion may be insufficient when mapping network variables using SNVT_count_inc because it has no units and often contains encoded values. In this case, consider disabling the automatic unit conversion, depending on the encoding of the value. To disable the automatic unit conversion, set the units for both the field and display units to the same selection. Any Custom Scale/Offset calculation is still performed, but the unit’s conversion is ignored.

For example, to add a multiplier of 0.1 to a mapped network variable that normally shows 831 to represent 83.1 Deg F, add a Custom Scale/Offset formula of x*0.1 and set both the Field Units and the Units to Deg F.

Note: By default, the math performed by the Custom Scale/Offset is applied after the unit conversion. To apply the conversion first, place a B in the field before the formula.

In the Focus tab, if a conversion sends a value that is outside the Min/Max range, the point displays ??? and Unreliable in the header. The Reliability attribute located under Status indicates if the value is above or below the range. In Figure 1, the Reliability attribute indicates Out of range high, or a value above the maximum.

Figure 1. Focus tab

Figure 2 shows the Hardware tab with Custom/Scale Offset formula (X*.1). Using this formula changes the original value from 1418 to 141.8. The Out of Range and Unreliable errors no longer appear, since the point is now in range.

Note: The point status in the header only changes when a COV occurs. The units and reliability change shortly after the units change or the point goes into range.
Figure 2. Hardware tab

Unreliable may appear if the point is out of range (high or low) or the Hardware and Display units are not valid (gallons per minute [GPM] to Deg F and other invalid unit conversions).