Localization and Units

Fieldbus data may be used by a variety of applications in different environments and platforms, therefore one important goal in an Internet/fieldbus design is interoperability. Two important aspects are localization and units which accompany fieldbus data.

Localization primarily deals with translation into other languages⁶⁷. For instance, it may be appropriate to provide textual information which accompanies certain fieldbus data, such as ``Air Condition Status''. This textual information may be then be offered in various languages, such as ``Status der Klimaanlage'' in German. Clients which request fieldbus data from Internet/fieldbus gateways with localization support may then specify their ``Locale'' and retrieve localized fieldbus data this way⁶⁸.

Fieldbus data may also be provided in different units, such as a mass that may be in kilograms (kg) or in pounds (lb). Client applications have to be informed about units in which data is provided, so that the data can be processed correctly. Client applications may however depend on other units than the fieldbus itself offers. Therefore it may be appropriate to offer data in other units than in its defaults, which may be done as follows:

SI Units:

One common way is to only provide and accept fieldbus data with a combination of the seven SI units. This strict approach is quite simple to implement⁶⁹ but may be inappropriate for certain clients that do not implement the SI system.

Implement a set of unit conversions:

This approach implements a variety of units together with conversion functions in the gateway. Clients may then retrieve fieldbus data in one of these units.

Conversion functions:

Another way would be to implement SI units and accept conversion functions in fieldbus requests which can then be used to convert fieldbus data from SI units to the the specified unit. This conversion function is commonly described as a linear equation, such as $unit = (A+Bx) / C+Dx)$, where $A, B, C, D$ are arbitrary constants. Although most common units can be expressed this way, units with non-linear conversion functions don't fit into this model. The approach is further described by [OBIX] and [BART04].