When an alarm occurs, the Analog Alarm block generates an alarm and closes the associated contact (digital block). For example, when a High alarm occurs, the block sends a High alarm to the alarm destinations defined in the SCU and closes the contact you defined for the High alarm.
The block handles each type of alarm independently of other alarms. For example, if a Rate of Change (ROC) and a High alarm occur simultaneously, the Analog Alarm block generates both alarms and closes the specified contacts.
Some alarm transitions generate only one alarm. For example, when the Analog Alarm block goes from a normal state to a High High alarm, the contacts for the High and High High alarms close. The block generates a High High alarm and suppresses the High alarm. Transitions from a normal state to a Low Low alarm work in a similar manner. The Low and Low Low alarm contacts close. The block generates a Low Low alarm and suppresses the Low alarm.
Block Mode
The block's Out Mode determines how it responds to certain alarm transitions. Possible Out Mode values are:
|
Entry |
Description |
|
Acknowledge |
Opens the digital block when the operator acknowledges the alarm. |
|
Return |
Opens the digital block when the alarm is cleared. |
|
All Clear |
Opens the digital block when the alarm is acknowledged and cleared. |
|
Never |
Does not open the digital block. |
For example, when the block's Out Mode is set to Return and the current alarm changes from a Low or Low Low alarm to a High High alarm, the block generates a High High alarm only after opening the Low and Low Low contacts and closing the High and High High contacts. Similarly, if the Out Mode is set to Return and the current alarm changes from a High or High High alarm to a Low Low alarm, the High and High High contacts open, the Low and Low Low contacts close, and the block generates a Low Low alarm.
The block's Out Mode also determines how the block responds to less abrupt alarm transitions. For example, when the current alarm changes from a High High alarm to a High alarm, the High High alarm contact opens depending on the alarm's mode. The block does not generate a High alarm and the state of the High alarm contact is unchanged because the block is still in an High alarm state. The transition from Low Low to Low works in a similar manner. The Low Low alarm contact opens, depending on the alarm's mode, the block does not generate a Low alarm, and the state of the Low alarm contact is unchanged. You can configure the Analog Alarm block to generate an alarm for either transition, by using a short re-alarm time for the High and Low alarm.
Using Contacts
You can specify the same contact for multiple alarms. When you do this, the contact closes when any alarm condition that uses the contact occurs. The contact remains closed until each alarm returns to normal. For example, if the High and High High alarms use the same contact, and a High High alarm occurs, the contact remains closed until the block's value falls below the High alarm limit.
