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Chapter 4
Block Reference
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National Instruments Corporation
4-7
UNACKNOWLEDGED
The
UNACKNOWLEDGED
subfield indicates the acknowledgment state of the
alarm or event. A host application typically acknowledges the
unacknowledged alarm when an operator sees and acknowledges the alarm.
unacknowledged alarm when an operator sees and acknowledges the alarm.
ALARM_STATE/UPDATE_STATE
The current state of the alarm or event can be determined through the
ALARM_STATE
(for alarms) or
UPDATE_STATE
(for events) subfield of the
alarm or event parameter. This parameter shows the active/clear state of
alarms and the reported/unreported state of both alarms and events.
alarms and the reported/unreported state of both alarms and events.
The first piece of state information in the state field is the active/clear state
of the alarm. An alarm is considered to be active when the alarm condition
is detected to be true. In the case of a limit alarm, the alarm is active when
the process variable, such as the temperature being measured, is beyond the
limit. The alarm state clears when the process variable returns within the
limit plus a hysterisis factor specified in the
of the alarm. An alarm is considered to be active when the alarm condition
is detected to be true. In the case of a limit alarm, the alarm is active when
the process variable, such as the temperature being measured, is beyond the
limit. The alarm state clears when the process variable returns within the
limit plus a hysterisis factor specified in the
ALARM_HYS
parameter.
All blocks have one alarm known as the Block alarm. The Block alarm is
considered active when any block error conditions (in the
considered active when any block error conditions (in the
BLOCK_ERR
parameter) are true. The Block alarm clears out when the last block error
condition goes false.
condition goes false.
For all alarms, the alarm condition is checked during each block execution.
Events, on the other hand, are not considered to be active or clear, but
simply one-time notifications.
Events, on the other hand, are not considered to be active or clear, but
simply one-time notifications.
The second piece of state information in the state field is the reported status
of the alarm or event. When an alarm or event condition is reported to the
host computer, an event notification message is broadcast on the bus if the
alarm has a priority greater than 1. For alarms without priority parameters
(Block alarms and events), the priority defaults to 2 and is always reported.
To confirm the receipt of the event notification, the host computer responds
with a confirmation message (different from the acknowledgment message
discussed above). Until the device receives the confirmation message, the
alarm or event is considered to be unreported. In the case where a block has
no alarm linkage, the device waits to report the alarm or event until the
linkage is established. If an alarm is unable to be reported to a host, the
Active/Clear state of the alarm stays constant until the alarm can be
reported.
of the alarm or event. When an alarm or event condition is reported to the
host computer, an event notification message is broadcast on the bus if the
alarm has a priority greater than 1. For alarms without priority parameters
(Block alarms and events), the priority defaults to 2 and is always reported.
To confirm the receipt of the event notification, the host computer responds
with a confirmation message (different from the acknowledgment message
discussed above). Until the device receives the confirmation message, the
alarm or event is considered to be unreported. In the case where a block has
no alarm linkage, the device waits to report the alarm or event until the
linkage is established. If an alarm is unable to be reported to a host, the
Active/Clear state of the alarm stays constant until the alarm can be
reported.