Until they have been forwarded to all the pre-configured alarm recipients, all alarm messages are normally stored in the automation station.
Each automation station has its own alarm queue for this purpose. Each incoming alarm and each system event is entered in the queue. An entry remains in the queue until the alarm or system event has been sent with a confirmed event notification to all recipients listed in the notification class object, and until the relevant acknowledgements have been received.
If the queue is full to overflowing, the oldest entries are deleted automatically, and a system event message is generated. Entries are deleted irrespective of alarm priority.
Alarm queuing has no effect on the alarm state of the alarm source.
Alarms destined for a temporary alarm recipient are not saved in the automation station. If a temporary alarm recipient can no longer be reached, the address of the recipient concerned is removed from the [RecpList] of the device object.
BACnet device object properties
The queuing of alarms is controlled by the following BACnet properties in the device object of the Desigo PX automation stations. These properties are not mapped to a function block, and can therefore only be viewed and modified online.
Buffer size [BufSize]
This BACnet property defines the maximum number of entries which can be saved in the queue.
A new value will only be accepted if it is greater than the record count [RecCnt].
Buffer size [BufSize] of the alarm queue.
- Default = 100 (PXC) or 150 (PXR)
- Range = 10…500 depending on the available memory space
Record count [RecCnt]
This BACnet property represents the number of entries currently stored in the queue.
The alarm queue can be deleted by writing the value 0 to this property. A write of a value not equal to 0 results in an error message.
If the queue is deleted, this information is entered as a system event in the queue and transmitted to the receivers. This causes the value to change to 1 as soon as [RecCnt] is set to zero.
Notification threshold [NotifThd]
This BACnet property defines the dial-out threshold, and the number of alarms to be deleted in the event of a queue overflow.
If [RecCnt] is greater than or equal to [NotifThd], a connection is established with any alarm recipients connected by cable (modem) which are destined to receive alarms and events from the queue. The connection is established provided that the remote alarm recipient concerned is listed in the notification class object.
The message threshold also defines how many alarms are to be deleted in the event of a queue overflow. As many alarm entries are deleted as necessary until [RecCnt] is equal to [NotifThd]. This function does not distinguish between local and remote alarm recipients in the notification class object.
To avoid deleting too many alarms, it is recommended that [NotifThd] be set to approximately 80% of the [BufSize].
Alarm queue message threshold [NotifThd]:
- Default = 80 (PXC) or 130 (PXR)
- Range = 5…495
Alarm priority limit [PrlmAlm]
This BACnet property defines another independent threshold for dial-out.
An alarm priority which is less than or equal to [PrlmAlm] results in a connection with any alarm recipients connected by cable (modem) which are destined to receive alarms and events from the queue. The connection is established provided that the remote alarm recipient concerned is entered in the list of Notification Class objects.
Alarm priority limit [PrlmAlm]:
- Default = 2 (HighPrioAlarm or UrgentAlarm)
- Range 0…255
If the notification class object contains only local alarm recipients, then the optimum results are achieved by use of the default values for control of alarm queuing. The default values should therefore not be changed.
If the notification class object contains remote alarm recipients, then it may be appropriate to modify the default values for control of alarm queuing.
The values for [NotifThd] and [PrlmAlm] determine when a remote cable connection (modem) is to be established, in order to inform the user of the occurrence of alarms.
If low-priority alarms are to be forwarded immediately, the [PrlmAlm] value must be increased (the higher the number, the lower the alarm priority). The value for [NotifThd] must not be modified.
The value for [NotifThd] can be reduced, however, in cases where a connection is to be established when there is a smaller number of alarms in the alarm queue. It is important to ensure, however, that the difference between [BufSize] and [NotifThd] does not become too great, as this is the value that controls the deletion of alarms in the event of a queue overflow.
Note that modifying these values also affects connection costs.
It takes time to establish a connection by cable (modem). If it is likely that further alarms will occur during this time, thereby causing the queue to overflow, the difference between [BufSize] and [NotifThd] should be increased.
The following settings are recommended:
- Buffer size [BufSize] = 120
- Notification threshold [NotifThd] = 80
In cases of doubt, the default values should be left unchanged.