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Instruction Manual
IM-106-880C, OI
September 2009
Hazardous Area OCX 8800
7-30
Advanced Features The AI function block provided with Fisher-Rosemount fieldbus devices
provides added capability through the addition of the following parameters:
ALARM_TYPE – Allows one or more of the process alarm conditions
detected by the AI function block to be used in setting its OUT_D parameter.
OUT_D – Discrete output of the AI function block based on the detection of
process alarm condition(s). This parameter may be linked to other function
blocks that require a discrete input based on the detected alarm condition.
VAR_SCAN – Time period in seconds over which the variability index
(VAR_INDEX) is computed.
VAR_INDEX – Process variability index measured as the integral of average
absolute error between PV and its mean value over the previous evaluation
period. This index is calculated as a percent of OUT span and is updated at
the end of the time period defined by VAR_SCAN.
Application Information The configuration of the AI function block and its associated output channels
depends on the specific application. A typical configuration for the AI block
involves the following parameters:
Channel
If the device supports more than one measurement, verify that the selected
channel contains the appropriate measurement or derived value.
L_TYPE
Select Direct when the measurement is already in the engineering units that
you want for the block output.
Select Indirect when you want to convert the measured variable into another,
for example, pressure into level or flow into energy.
Select Indirect Square Root when the block I/O parameter value represents
a flow measurement made using differential pressure, and when square root
extraction is not performed by the transducer.
Scaling
XD_SCALE provides the range and units of the measurement and
OUT_SCALE provides the range and engineering units of the output.
Application Examples Temperature Transmitter
Situation
A temperature transmitter with a range of –200 to 450°C.
Solution
Table 7-21 lists the appropriate configuration settings, and Figure 7-8
illustrates the correct function block configuration.
Table 7-21. Analog Input
Function Block Configuration for
a Typical Temperature
Transmitter
Parameter Configured Values
L_TYPE Direct
XD_SCALE Not Used
OUT_SCALE Not Used