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Appendix
B-1
B Analog Input (AI)
Function Block
The Analog Input (AI) function block processes field device
measurements and makes them available to other function blocks. The
output value from the AI block is in engineering units and contains a
status indicating the quality of the measurement. The measuring
device may have several measurements or derived values available in
different channels. Use the channel number to define the variable that
the AI block processes.
The AI block supports alarming, signal scaling, signal filtering, signal
status calculation, mode control, and simulation. In Automatic mode,
the block’s output parameter (OUT) reflects the process variable (PV)
value and status. In Manual mode, OUT may be set manually. The
Manual mode is reflected on the output status. A discrete output
(OUT_D) is provided to indicate whether a selected alarm condition is
active. Alarm detection is based on the OUT value and user specified
alarm limits. Figure B-1 on page B-4 illustrates the internal
components of the AI function block, and Table B-1 lists the AI block
parameters and their units of measure, descriptions, and
index numbers.
OUT = The block output value and status
OUT_D = Discrete output that signals a selected
alarm condition
OUT_D
AI
OUT
FIELDBUS-FBUS_31A
TABLE B-1. Definitions of Analog Input Function Block System Parameters.
Parameter
Index
Number
Units Description
ACK_OPTION 23 None Used to set auto acknowledgment of alarms.
ALARM_HYS 24 Percent The amount the alarm value must return within the alarm limit before the associated
active alarm condition clears.
ALARM_SEL 38 None Used to select the process alarm conditions that will cause the OUT_D parameter to
be set.
ALARM_SUM 22 None The summary alarm is used for all process alarms in the block. The cause of the
alert is entered in the subcode field. The first alert to become active will set the
Active status in the Status parameter. As soon as the Unreported status is cleared
by the alert reporting task, another block alert may be reported without clearing the
Active status, if the subcode has changed.
ALERT_KEY 04 None The identification number of the plant unit. This information may be used in the host
for sorting alarms, etc.