Ashrae Journal - November 2008 - B13

crement property. This has the same effect as if you wrote to Present_Value with Present_Value + Step_Increment. STEP_DOWN means to decrease the output by an amount specified in the Step_Increment property. This has the same effect as if you wrote to Present_Value with Present_Value – Step_Increment. STEP_UP_BY increases Present_Value and specifies the increment amount as a parameter in the operation. STEP_ DOWN_BY decreases Present_Value and specifies the increment amount as a Figure 7: Blink warning timing. parameter in the operation. An important lighting command is RELINQUISH. This has lighting, sharing the same range of channel numbers could be the effect of writing a NULL value (relinquish) to the Pres- limiting to some BACnet applications. For that reason, the ent_Value at the priority specified by Lighting_Command_Pri- CH object and WriteGroup have an additional concept called ority, in effect releasing the control of the Present_Value at that control groups. Control Groups allow for the dynamic reassignment of channels to specific groupings, which is useful for priority level. Often there are multiple dimmer outputs that, while they are dynamic room conἀguration applications such as ballrooms and conference suites. separate and discrete outputs that could be These concepts allow WriteGroup to proindividually addressed, under some circumvide a compact and efficient representation stances want to behave as if they were one for transmitting change requests. While not logical output (Figure 8 ). In BACnet, this quite as lean as the DMX protocol that is application is modeled as a logical channel popular with theatrical lighting, WriteGroup using a channel object (CH). A channel object can provide high speed transmission of has a Present_Value that can feed forward complex lighting commands while making to multiple individual object property desonly modest demands on BACnet network tinations. The idea is that any time the CH instance Present_Value property is written, bandwidth. that value is rewritten to a specific collection Although the original BACnet standard of other object properties. These objects can objects have been used successfully for live in the same device that contains the CH lighting applications since 1995, these new object, or in other BACnet devices, allowing developments in extending the BACnet stanthe channel object to use an intermediary dard with some lighting-centric features are device to manage the forwarding operation. a welcome addition. Figure 8: Lo gical c hannel using a Because of latency issues, CH objects most After a successful vetting through channel object (CH) feeding forward often restrict their use to redistributing the several public reviews, we expect to see to multiple LO objects. value to objects in the same device. products emerging in the marketplace that Although there are various applications for CH objects, take advantage of new BACnet lighting features. Lighting often we want to be able to send a single write request (aimed applications are highly dynamic and are taking on an increasat a specific channel for example) that multiple devices can ingly important role in energy savings, green initiatives, receive and act upon at the same time. BACnet lighting ap- and building environment and ergonomics. The Lighting plications use the special WriteGroup service to send these Application Working Group is tackling additional new kinds of requests. territory in architectural and theatrical lighting, including Unlike regular WriteProperty requests that only change a color and motion control. With the shift from incandescent property of an object in one device; the WriteGroup can change to LED-based lighting in the near future, the issue of how properties in objects in multiple devices at the same time. To to control and interoperate across lighting systems and do this, the WriteGroup is broadcast in a way that allows many building automation systems is timely and critical. What devices to hear the same request simultaneously. For this to better way to help them along than using the international make sense and not cause chaos, all of the listening devices standard BACnet? must use the same assignment of channel numbers to mean the Author’s Note same logical thing. This article is an abbreviated version of a white paper published The range of channel numbers is restricted from 1 to 65535. In a large facility with other types of applications besides by the SSPC135 Lighting Applications Working Group. November 2008 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 B13

Ashrae Journal - November 2008

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