Ashrae Journal - November 2008 - (Page B12)

• 1% = Dimmest; and • 100% = Brightest. When two or more controllers attempt to write to the same property of the same object, there is the potential for conflict from a control perspective. BACnet provides a prioritization mechanism that allows you to assign relative importance to each potential writer so that the system can decide who is most important when multiple writes occur to the same output. In these cases, the written value includes a priority level from one (most Figure 4: Stepping is the procedure of incrementally increasing or decreasing lighting level important) to 16 (least important). The in ἀxed steps (for example when a wall panel  or  button is pressed). object remembers each written value at each priority level and writers are also allowed to relinquish of time. Second, each operation requires additional parameters their control of an output, which causes the next most important to completely specify the desired effect and the parameters are value to take effect. This kind of output is called commandable, different for each kind of operation. The special property Lighting_Command may be written with which means that it can be changed and the changes can be a compound value that specifies a unique lighting operation and coordinated among multiple sources. Because it’s commandable, the usual prioritizing behavior can initiates that operation. When Lighting_Command is written, be expected for writes to an LO Present_Value. Some optional any in-progress operation is canceled first. Lighting_Command properties allow the Present_Value to be clamped within a range is not commandable and no attempt is made to synchronize when written with a nonzero value (Figure 6). For example, if multiple writers, so the most recent write wins. However, when the property Min_Pres_Value = 15, and Max_Pres_Value = 80 lighting operations occur, they affect the Present_Value at the then when Present_Value is written with 1, the Present_Value priority level specified in the Lighting_Command_Priority property, and the Present_Value is comand output are clamped to 15% instead. mandable. When Present_Value is written with 100, The simplest operation is called STOP. Present_Value and output are clamped to 80%. Any previously initiated operation must Blink warning is intrinsically supported stop regardless of whether it has comin the LO object. When Present_Value is pleted yet. For example, we could initiate Figure 5: The LO object is similar to an AO written with a 0 value, which intuitively representing a 0% to 100% lighting intensity a fade from 100% to 50% over 10 seconds, means “turn off ”, the output turns off output. but four seconds into the execution of for Blink_Time (usually brief) if the the fade we could receive a STOP that Blink_Time property has a nonzero time would cancel further fading leaving the value, indicating that blink warning is output at 80%. desired. The next simple operation is called Normally, the output is turned on GOTO_LEVEL, which has a single paagain after the Blink_Time and remains rameter for the desired target level. This on for a while before turning off again. Figure 6: Pr esent_Value can be clamped has the same effect as writing the desired This period of time is called Off_Delay within a ra nge w hen written with a non- target level directly to Present_Value. (Figure 7 ). In some cases, even though zero value. There are two operations for fading. a given output usually wants to support FADE_TO specifies a target level to fade blink warning, it may be desirable to force the output off im- to and the period of time over which the fade occurs uses a mediately and ensure that it stays off, in effect trumping the default period of time specified by the Fade_Time property. blink warning feature. For example, in a conference room or FADE_TO_OVER includes the target level as well as the fade auditorium when a presentation is being made, we may want time as parameters to the operation. to turn the lights off immediately and no warning is required There are two types of ramping operation. RAMP_TO at this moment. For those applications, the priority used when specifies a target level and the Ramp_Rate property specifies a writing to the Present_Value can be configured so that blink default rate of change for ramping. When the level is achieved, warning does not occur. the ramping stops. RAMP_TO_AT_RATE specifies both the The concepts of fading, ramping, and stepping are special for target level and the rate. two reasons. First, these ideas involve some operation that is There are four types of stepping operation. STEP_UP means initiated by writing to a property, but is carried out over a period to increase the output by an amount specified in the Step_InB12 B A C n e t ® To d a y | A S u p p l e m e n t t o A S H R A E J o u r n a l November 2008

Table of Contents Feed for the Digital Edition of Ashrae Journal - November 2008

Ashrae Journal - November 2008 Contents Commentary Industry News Meetings and Shows Feature Articles Reducing Data Center Energy Consumption Building Sciences Feature Articles Sustainable Nature Center Emerging Technologies Special Products Washington Report Special Supplement: BACnet® Today BACnet® Goes Green Seeing the Light with BACnet® BACnet® on Campus BACnet® for Net Zero Analyzing BACnet® BACnet® at Xerox Campus BACnet® in Europe BACnet® for Green Library BACnet® at the University of Minnesota: Flexibility, Interoperability InfoCenter Product Showplace Classified Advertising Advertising Index

Ashrae Journal - November 2008

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