Emerson Fisher FIELDVUE DVC6000f, Instruction Manual

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AI Function Block
December 2009 4-129
Figure 4-24. Analog Input (AI) Function Block
OUT_D
OUT
OUT
OUT_D
= THE BLOCK OUTPUT VALUE AND STATUS
= DISCRETE OUTPUT THAT SIGNALS A SELECTED
ALARM CONDITION
FIELDBUS-FBUS_31A
Analog Input (AI) Function Block
Overview
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 [8]) reflects the process variable (PV
[7]) value and status. In Manual mode, OUT [8] may
be set manually. The Manual mode is reflected on the
output status. A discrete output (OUT_D [37]) is
provided to indicate whether a selected alarm
condition is active. Alarm detection is based on the
OUT [8] value and user specified alarm limits. Figure
4-26 illustrates the internal components of the AI
function block, and table 4-56 lists the AI block
parameters and their units of measure, descriptions,
and index numbers.
Analog Input Block Modes
The AI function block supports three modes of
operation as defined by the MODE_BLK [5]
parameter:
Manual (Man) The block output (OUT [8]) may
be set manually.
Automatic (Auto) OUT reflects the analog input
measurement or the simulated value when simulation
is enabled.
Out of Service (OOS) The block is not
processed. FIELD_VAL [19] and PV [7] are not
updated and the OUT [8] status is set to Bad: Out of
Service. The BLOCK_ERR [6] parameter shows Out
of Service. In this mode, you can make changes to all
configured parameters. The target mode of a block
may be restricted to one or more of the supported
modes.
Alarm Detection
A block alarm will be generated whenever the
BLOCK_ERR [6] has an error bit set. The types of
block errors for the AI block are defined in table 4-54.
Process Alarm detection is based on the OUT [8]
value. You can configure the alarm limits of the
following standard alarms:
High (HI_LIM [28])
High high (HI_HI_LIM [26])
Low (LO_LIM [30])
Low low (LO_LO_LIM [32])
In order to avoid alarm chattering when the variable is
oscillating around the alarm limit, an alarm hysteresis
in percent of the PV [7] span can be set using the
ALARM_HYS [24] parameter. The priority of each
alarm is set in the following parameters:
HI_PRI [27]
HI_HI_PRI [25]
LO_PRI [29]
LO_LO_PRI [31]
Alarms are grouped into five levels of priority, as
shown in table 4-52.
Table 4-52. AI Function Block Alarm Priorities
Priority
Number
Priority Description
(1)
0
The priority of an alarm condition changes to 0 after the
condition that caused the alarm is corrected.
1
An alarm condition with a priority of 1 is recognized by the
system, but is not reported to the operator.
2
An alarm condition with a priority of 2 is reported to the
operator, but generally does not require operator attention
(such as diagnostics and system alerts).
3
−
7
Alarm conditions of priority 3 to 7 are advisory alarms of
increasing priority.
8
−
15
Alarm conditions of priority 8 to 15 are critical alarms of
increasing priority.
1. The priority classes “advise” and critical” have no relationship to Plant Web Alerts.
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