BACnet: Notification Class Objects

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