Analog Alarm Block

Purpose

The Analog Alarm (AA) block sends and receives analog data from the I/O driver or OPC server to provide alarm control. Using this block you can suspend alarms and define limits and priorities for each alarm. The block can also wait a specified time interval before issuing an alarm, close a contact when an alarm occurs, and automatically reissue and acknowledge alarms.

Features

The Analog Alarm block:

• Is a primary block.
• Can be used as a stand-alone block.
• Generates alarms when values are outside of defined limits.
• Sends and receives values from an I/O driver or OPC server when placed in Automatic mode.
• Receives values from operator input in the iFIX WorkSpace, from scripts, from Program blocks, or from Easy Database Access programs when operating in Manual mode.
• Can be used in a chain with exception-based or time-based processing.
• Can be configured using Continuous Output that allows blocks to attempt to write the contacts with every scan, even if the value being written in unchanged. Without Continuous Output, the block only attempts to write to the defined contact tag when a value has changed and it needs to be written to the PLC. (In this instance, the write is a one-time attempt, so if it fails, the write will not be retried until the block needs to write a new value.)

NOTE: Because the contact mode of “Never” does not reset the contact, the Continuous Output option is not supported for this contact mode.

• Can be configured using the Suppress COMM Alarm option, which allows you to separate the original alarm condition from the COMM alarm, and return the AA block to the same state as it was prior to a COMM alarm. If you disable the Suppress COMM Alarm option,  AA blocks handle one alarm at a time. (As a result, it is possible that acknowledgement of a COMM alarm could cause the ACK bit in the PLC to be written, and the original alarm condition, if already acknowledged, could re-alarm.)

• Can be configured to enable the Event messaging (Suspend mode), which applies suppression behavior to disable alarm processing. When the tag is in suspend mode, the Alarm state is set to OK, the Alarm is an alarm message only and therefore, does not appear in the alarm summary. Alarm processing continues with each alarm state transition recorded in the alarm loggers but does not display in the alarm summary. The alarm state contact (tag) is not processed.

• Can use linear scaling on values received. The following formula is used to determine the linear scale value:

The equation for a line is Y = mX + b

Where Y is the scaled output value, X is the raw value from the PLC, m is the slope of the line and b is the y intercept.

The slope for our linearization is defined by output range divided by the input.

m  =  (ScaleHigh - ScaleLow ) / (RawHigh - RawLow)

The y intercept is determined by using the low values and the slope.

b = ScaleLow - m(RawLow)

In the analog block this results in the block's current value is equal to the driver value times the slope plus the y intercept.

X_CV = (IO value)m + b  

The driver value equates to the block current value minus the y intercept divided by the slope.

IO Value = (X_CV – b) / m

See Also

Typical Uses

Example

Fields