CHANGE_OF_STATE and OUT_OF_ RANGE. "Information alerts" are not a replacement for BACnet alarms. Alerts are suited for sending maintenance notifications to another device. BACnet Objects for Lighting The original plan to meet the needs of the lighting industry was to extend the existing BACnet objects like the Binary and Analog Output object types. It soon became clear that this was the wrong approach and that new lighting centric objects were needed. Lighting specific strategies such as "blink-warn," ramping up to a specific brightness level or fading the light to off could not be easily accommodated by enhancing the standard existing BACnet objects. Lighting Output Object Type Notification-Client 1 Notification-Server 1 Notification-Client 2 Notification-Server 2 Notification Forwarder Object Notification-Client 3 Notification-Server 3 Notification-Client 4 Figure 1: Example of multiple Notification-servers routing to multiple NotificationClients. (Source: Figure 12-7 from SSPC 135-2012). Present_Value=100%, Pr=9 Priority 1 7:00 a.m. Lighting Command = WARN_RELINQUISH Pr=9 Blink-Warn Notification 6:00 p.m. 6:10 p.m. 0% The Lighting Output object type defines a standardized object whose prop100% Normal Office Hours erties represent the externally visible Priority 9 0% characteristics of a lighting output and includes dedicated functionality specific to lighting control that would otherwise Priority 16 0% require explicit programming. The lighting output is analog in nature. Relinquish Defaul = 0.0% The physical output level, or nonnormalized range, is specified as the linearized Figure 2: Daily schedule with Blink-Warn example. (Source: Figure 12-14 from SSPC 135-2012). percentage (0% to 100%). The actual value may range from 5% to 85%. These values off or issue a notification in some other manner. can be defined as a subset of physical output levels defined by The following example illustrates how a Lighting Output obMin_Actual_Value and Max_Actual_Value. ject type may be used in a typical office scenario. The level of the lights can be changed directly to an absolute Prior to 7 a.m. the lights are off as the Lighting Output oblevel by writing to the Present_Value. This property is com- ject type is being controlled at the relinquish default value of mandable and uses BACnet's 16 level priority array mechanism 0.0% or off. At 7:00 a.m. a scheduler (e.g., a BACnet Schedule to arbitrate between multiple writers to the lighting output. This object or other automated process) turns the physical lights on notion of having more than one device writing to an object is by writing 100% to the Present_Value property at priority 9. At not used much in the HVAC world but is commonplace in the 6 p.m. a WARN_RELINQUISH lighting command is executed lighting world. The light might start with defined brightness at priority 9, which causes an immediate blinkwarn notificalevel during occupancy. This level could be reduced by a day- tion to occur but leaves the lights on until the egress timer has lighting sensor if the sun is shining into the office space. There expired. Assuming a 10 minute (600 seconds) egress time is may be a lighting load shedding strategy controlled by a Load specified, the value at priority 9 is relinquished at 6:10 p.m., Control object that could then reduce the lighting output. The and the Present_Value returns to 0%. This scenario is shown local user normally has the final say and may set the brightness in Figure 2. of the room lights through a local controller on the wall. The Lighting_Command is a writeable property that pro- Channel Object Type vides additional lighting functionality with special lightingThe Channel Object was needed for the lighting industry to specific functions such as ramping, stepping and fading. The allow for writing a lighting brightness value to as many as dozLighting_Command also provides a blink-warn mechanism to ens of individual lights in many lighting controllers quickly. notify room occupants that the lights are about to turn off. The The existing WritePropertyMultiple service requests could blink-warning mechanism is internal to the Lighting Output be used but this approach would not provide the fast response object type and may cause the physical lights to blink on and needed for the lighting industry. B10 B A C n e t ® t o d Ay A n d t h e S m A rt G r i d | 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 n ovem Ber 2013 ashrae.org