The Command object is a very important part of the operation of a BACnet device. The Command object writes to a group of object properties through action code(s) within the Present_Value property. When this writing occurs, the Command object will start a sequence of actions in the BACnet device(s).

The Command object like the object in the previous article by the same author uses the four properties required by the BACnet standard. In addition, five more important properties are included in the operation of this object.

The following table will list the properties and datatypes of the Command object from the BACnet standard:

The Command object can be a powerful aspect with beneficial applications, but at the same time this object can cause confusion and problems for the developer. Certain issues must be observed concerning the Command object:

• Proper configuration of the Command object.
• Proper device configuration concerning the Command object.
• Closely monitor the In_Process property of the Command object, due to the possibility of the object commanding itself.

The following example from the BACnet standard will reveal the Command object structure from an actual building automation application. Specifically, an elevator subsystem where a floor of the building is shut off when in the unoccupied mode:

Object_Identifier = (Command, Instance 1)
Object_Name = "ZONE43CONTROL"
Object_Type = COMMAND
Description = "Fourth Floor, West Wing Office Suite"
Present_Value = 2
In_Process = FALSE
All_Writes_Successful = TRUE
Action = {((,(Analog Value, Instance 5), Present_Value,,65.0, 8,,TRUE,TRUE),
(,(Binary Output, Instance 3), Present_Value,,INACTIVE,8,1,TRUE,TRUE),
((Device, Instance 1), (Binary Value, Instance 1),Present_Value, ,INACTIVE, 8,1,TRUE,TRUE)), ((,(Analog Value, Instance 5), Present_Value,,72.0,
8,2,TRUE,TRUE), (,(Binary Output, Instance 3), Present_Value,
,ACTIVE,8,,TRUE,TRUE), ((Device, Instance 1), (Binary Value, Instance 1),Present_Value,,ACTIVE, 8,,TRUE,TRUE))}
Action_Text = {"Unoccupied", "Occupied"}

Written by: Scott Cosby

© Chipkin Automation Systems 2007

The BACnet Notification object is a BACnet Object that is used to send event notifications within a BACnet System. When a device has a problem or a fault, such as the present value of a property rise above a predetermined level, a notification object would be created to inform the rest of the BACnet system that there is something wrong. There are several way of sending this type of message to other devices with different priorities and different destination.

In addition, the notification class prioritizes events according to these transition definitions, and may involve the acknowledgment from a human user:

• TO-OFFNORMAL
• TO-FAULT
• TO-NORMAL

A device that is used to check the water level of a boiler might send a notification object with the priority of TO-OFFNORMAL if the water level is below a certain point, or TO-FAULT if the boiler i
s dry, or TO-NORMAL when water is added to the boiler and the water level returns to a normal level.

The primary reason for prioritization is to offer the user a way to ensure that alarms or events with critical time constraints are not delayed. The range of priorities includes 0 to 225 with the lower number (0) as a high priority than a high number (255).

The notification classes may specify a list of destinations, each of which can be defined by time, day of week, and handling type. The destination also defines the receiver device to that receives the notification and a process within the device. The processes are indicated by numerical handles that are only understood by the destination device. The handles are considered to be a local administration decision. The destination device is specified by the BACnet object “identifier” or through its address. Three types of addresses can be used:

• Node Address
• Multicast Address
• Broadcast Address

The destination further indicates whether the notification should be sent using a confirmed or unconfirmed event notification.

The following example was obtained from the 2004 BACnet Standard to illustrate the defining structure of a notification class object for critical system alarms, including a time-dependent receiver list:

Written by: Scott Cosby

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© Chipkin Automation Systems 2007