ASHRAE Journal - November 2010 - B20

in the second supply airstream of a complex, built-up air handler in the Smith building. The point naming method should be able to handle complex devices such as built-up air handlers. The particular air handler in the example has multiple supply airstreams, and each may have multiple coils and sensors. It is useful to be able to specify a particular point within the context of a named section of a complex device, in this case the second supply airstream. Table 4 (see Page B18) is the element table for this example. The resulting point name is: bldg-smith/air_dist/ahu-1/ sa-2/cc_chw/dat.

Anticipating the Need for Change
A point naming standard will likely require some new feature that was not originally anticipated. Designing flexibility into the point naming standard makes it easier to add new features. New point types and equipment types probably will need to be added to the standard over time, and more substantial changes may eventually be needed. For example, if the building engineers decide that they want to add site information to the point naming standard, is the point naming standard flexible enough to handle such a change? In our company’s point naming standard, we could add a new element for the site information because our standard does not set a limit on number of components in a point name. Returning to Example 1, the new site element would be added to the point name as follows: site-phoenix/bldg-alumni/plant/chlr-1/chwst_sp.

Point Name Implementation in BACnet Systems
The BACnet protocol is designed in such a way that useful point names can often be placed directly into the various BACnet objects in the devices in a control system. This is generally preferable to only storing point names in separate documentation such as spreadsheet tables; point names that are easily visible to system operators and system maintenance staff are more likely to be kept up to date. Although BACnet defines properties in every object that might be appropriate for the storage of customer-determined point names, some creativity may be required to work around the limitations of specific control products. Here are some of the building blocks that the BACnet protocol provides. • All BACnet objects: All objects contain an Object Name property, which contains a printable text string; the contents of the Object Name property must be unique among all objects within a single BACnet device. An object may also contain a Description property that contains a printable text string. • Device object: Every BACnet device is required to have one Device Object. Like all BACnet objects, the Device Object contains an Object Name property, which contains a printable text string; the contents of the Device Object’s Object Name property must be unique among all Device Objects within a BACnet internetwork. The Device Object may also contain a Location property that contains a printable text string.
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• Analog objects: All Analog Objects are required to have a Units property whose contents specify the engineering units of the object. Before developing a plan for the implementation of point names within specific BACnet compliant control products, it is necessary to understand certain characteristics of those control products. Here are three questions that should be answered: 1. Does the product support the Location property in the Device Object? If so, what is the maximum length of the contents of that property? 2. Does the product support the Description property in all objects? If not, which objects may contain a Description property? What is the maximum length of the contents of that property? 3. Can the contents of the Object Name property of all objects be modified? If so, what is the maximum length of the contents of the Object Name property, and are there any other restrictions on its contents (e.g., forbidden symbols or other syntactic rules)? Armed with this information, the building engineer can develop a method for mapping the components of the desired point naming standard into properties in the BACnet objects. Here is an example of a method that should work in many BACnetcompliant products: • The Location property of the Device Object will contain the location of the equipment (e.g., bldg-smith/rmbase_mech_rm) or the zone served by the equipment (e.g., bldg-biology/fl-2/z-lecture_hall). • The Object Name property of the Device Object will contain the equipment type and index (e.g., AHU-1). In case of a BACnet system that spans multiple buildings, it may be necessary to also include the building name to satisfy BACnet’s uniqueness requirement (e.g., bldg-smith/ ahu-1). • The Object Name property of the analog, binary and multistate objects will contain the point type, index, and other identifying information (e.g., z_t-3 or sa-2/cc_chw/ dat). If the object names of these objects are assigned automatically by the manufacturer and cannot be modified by the customer, then this information will be placed in the object’s Description property. • The Units property of each analog object will contain the correct engineering units for that object. • The Description property of the Device Object will be reserved for other information about the control device.

Conclusion
Point names can provide useful information to the people who install, operate and maintain building automation systems. Large organizations will particularly benefit from the use of a point naming standard that is tailored to the organization’s specific needs. The consistent application of a point naming standard can make it easier to deploy software for automated system performance analysis.
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ASHRAE Journal - November 2010

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