The BACnet Priority Array is a specific property type described by the standard as one of the command prioritization mechanisms.

The developer should be aware of the manner in which this property type functions concerning priorities within building control systems.

The Priority Array is further defined by the standard as a read-only property that consists of commands that categorized by priority levels of decreasing order. NULLs may also exist in this property type as well. The highest priority will reside with the lowest array index and a non-NULL value will represent the active command.

Prioritized commands are essentially those commands that are specific towards commandable properties. The following is a parameter example from the BACnet standard:

• Property Identifier: Commandable_Property
• Property Value: Desired Value
• Priority: Priority

The following table from the BACnet standard will examine the common command priorities of the priority array:

Command entities are assigned one of the sixteen priority levels. These assignments are usually a matter for the developer.

The table may include other commands such as the following building automation related applications mentioned in the BACnet standard:

• Temperature Override
• Demand Limiting
• Duty Cycling
• Scheduling

The following priority array structure example from an actual building automation application was obtained from the Synergy lighting control system developed by Lithonia
Lighting. The following are the local priorities used for the Synergy system:

#define PRIO_PRIORITY_ON 3 /* Synergy Unique */
#define PRIO_PRIORITY_OFF 4 /* Synergy Unique */
#define PRIO_MANUAL_OPERATION 8 /* only used with flash to find */
#define PRIO_NORMAL_OPERATION 10 /* Synergy Unique */
#define PRIO_PRIORITY_LOW 13 /* Synergy Unique */

Because the BACnet standard does not specifically indicate the automation application of lighting controls. The developers at Lithonia Lighting essentially made their own priority decisions when they implemented the BACnet protocol in the Synergy design.

The following paragraph was obtained from a PDF source that describes the priority assignment for effective operation of the lighting control system by Lithonia Lighting:

“It is strongly suggested that writes intending to turn lighting ON be made at command priority 10 and writes intending to turn lighting OFF write a NULL unless the intention is to forcibly override the normal functioning of the lighting controls.”

Written by: Scott Cosby
© Chipkin Automation Systems 2007

The BACnet standard describes two types of Life Safety objects. The two types of these objects include:

• Life Safety Point Object Type
• Life Safety Zone Object Type

The Life Safety Point object type is defined by the standard as those objects related to fire, safety, and security applications. The condition of this object type is determined by the following two factors:

• Mode
• State

The object mode is usually under the control of the device operator. The object state indicates the controller condition depending on the internal logic of the device.

Life Safety Point object applications can include the following automation related devices:

• Automatic Fire Detectors
• Sirens
• Pull Stations

The following table will present the properties and related datatypes of the Life Safety Point object from the BACnet standard:

The Life Safety Zone object type has similar conditional and property qualities as the Life Safety Point object type, but the focus is concentrated on a group of Life Safety Point and Zone objects.

The following table will present the properties and related datatypes of the Life Safety Zone object from the BACnet standard:

The developer should pay attention to the Mode and State qualities of both Life Safety object types.

The following structure example from the standard will examine both Life Safety object types from actual automation applications. Specifically, the Point object will focus on a building smoke detector. The Zone object will examine the fire zone within a building.

Life Safety Point Object Example:
Property: Object_Identifier = (Life Safety Point, Instance 2)
Property: Object_Name = “SMK3W”
Property: Object_Type = LIFE_SAFETY_POINT
Property: Present_Value = PREALARM
Property: Tracking_Value = PREALARM
Property: Description = “Floor 3, West Zone Smoke Detector”
Property: Device_Type = “Old Smokey model 123″
Property: Status_Flags = {TRUE, FALSE, FALSE, FALSE}
Property: Event_State = LIFE_SAFETY_ALARM
Property: Reliability = NO_FAULT_DETECTED
Property: Out_Of_Service = FALSE
Property: Mode = ON
Property: Accepted_Modes = {ENABLED, DISABLED, TEST}
Property: Time_Delay = 10
Property: Notification_Class = 39
Property: Life_Safety_Alarm_Values = (ALARM)
Property: Alarm_Values = (PREALARM)
Property: Fault_Values = (FAULT)
Property: Event_Enable = {TRUE, TRUE, TRUE}
Property: Acked_Transitions = {TRUE, TRUE, TRUE}
Property: Notify_Type = ALARM
Property: Event_Time_Stamps = ((23-MAR-95, 18:50:21.2), (*-*-*, *:*:*.*),(23-MAR-95, 19:01:34.0))
Property: Silenced = SILENCE_AUDIBLE
Property: Operation_Expected = RESET_ALARM
Property: Maintenance_Required = NONE
Property: Setting = 50
Property: Direct_Reading = 84.3
Property: Units = PERCENT-OBSCURATION-PER-METER
Property: Member_Of = ((Life Safety Zone, Instance 5))

Life Safety Zone Object Example:
Property: Object_Identifier = (Life Safety Zone, Instance 2)
Property: Object_Name = “SMK3″
Property: Object_Type = LIFE_SAFETY_ZONE
Property: Present_Value = PREALARM
Property: Tracking_Value = PREALARM
Property: Description = “Floor 3 Smoke”
Property: Status_Flags = {TRUE FALSE, FALSE, FALSE}
Property: Event_State = LIFE_SAFETY_ALARM
Property: Reliability = NO_FAULT_DETECTED
Property: Out_Of_Service = FALSE
Property: Mode = ON
Property: Accepted_Modes = {ENABLED, DISABLED, TEST}
Property: Time_Delay = 10
Property: Notification_Class = 39
Property: Life_Safety_Alarm_Values = (ALARM)
Property: Alarm_Values = (PREALARM)
Property: Fault_Values = (FAULT)
Property: Event_Enable = {TRUE, TRUE, TRUE}
Property: Acked_Transitions = {TRUE, TRUE, TRUE}
Property: Notify_Type = ALARM
Property: Event_Time_Stamps = ((23-MAR-95, 18:50:21.2), (*-*-*, *:*:*.*),(23-MAR-95,19:01:34.0))
Property: Silenced = UNSILENCED
Property: Operation_Expected = SILENCE_AUDIBLE
Property: Maintenance_Required = NONE
Property: Zone_Members = ((Life Safety Point, Instance 22),(Life Safety Point, Instance 23))
Property: Member_Of = ((Life Safety Zone, Instance 5))

Written by: Scott Cosby

© Chipkin Automation Systems 2007

The Pulse Converter object is described by the BACnet standard as a process monitor represented by counts or pulses.

The type of processes monitored can include those tasks in maintaining a building. Such as the following examples:

• Power Usage
• Water Usage
• Natural Gas Usage

The Pulse Converter object might represent a physical input. As an alternative, it might acquire the data from the Present_Value of an Accumulator object, representing an input in the same device as the Pulse Converter object.

The following table will present the actual properties and associated datatypes of the Group object from the BACnet standard:

The developer should pay close attention to a specific property in the preceding table. The specific property is the Adjust_Value property. This property allows the developer to adjust the Present_Value property by writing to the Adjust_Value property, which also causes an adjustment to the Count property as well.

The following example from the BACnet standard will present the Pulse Converter object structure from an actual automation application. This example will specifically focus on the utility usage in a building:

Property: Object_Identifier = (Pulse Converter, Instance 1)
Property: Object_Name = “Meter 5″
Property: Object_Type = PULSE_CONVERTER
Property: Description = “”
Property: Present_Value = 125.0
Property: Input_Reference = ((Accumulator, Instance 1), Present_Value)
Property: Status_Flags = {FALSE, FALSE, FALSE, FALSE}
Property: Event_State = NORMAL
Property: Out_Of_Service = FALSE
Property: Units = LITERS_PER_HOUR
Property: Scale_Factor = 0.5
Property: Adjust_Value = 500.0
Property: Count = 250
Property: Update_Time = (10-JUL-01,11:40:21.0),
Property: Count_Change_Time = (10-JUL-01,11:30:01.0),
Property: Count_Before_Change = 523
Property: COV_Increment = 10.0
Property: COV_Period = 3600
Property: Notification_Class = 5
Property: Time_Delay = 0
Property: High_Limit = 1000.0
Property: Low_Limit = 0.0
Property: Deadband = 0.0
Property: Limit_Enable = {FALSE, TRUE}
Property: Event_Enable = {TRUE, FALSE, TRUE}
Property: Acked_Transitions = {TRUE, TRUE, TRUE}
Property: Notify_Type = ALARM
Property: Event_Time_Stamps = ((12-JUL-01,18:50:21.2), (*-*-*,*:*:*.*), (12-JUL-01,19:01:34.0))

Written by: Scott Cosby

© Chipkin Automation Systems 2007