Emerson Rosemount 848T, Reference Manual, Page: 113 / 136

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Reference Manual
00809-0100-4697, Rev GA
Appendix D: Function Blocks
November 2014
Function Blocks
Modes
The AI Function Block supports three modes of operation as defined by the MODE_BLK
parameter.
Manual (Man)
The value of the block output (OUT) 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 and PV are not updated and the OUT status is set to
Bad: Out of Service. The BLOCK_ERR parameter shows Out of Service. In this mode,
changes can be made to all configurable parameters.
Alarm detection
A block alarm will be generated whenever the BLOCK_ERR has an error bit set. The types of
block error for the AI block are defined above.
Process alarm detection is based on the OUT value. Configure the alarm limits of the following
standard alarms:

High (HI_LIM)

High high (HI_HI_LIM)

Low (LO_LIM)

Low low (LO_LO_LIM)
To avoid alarm chatter when the variable is oscillating around the alarm limit, an alarm
hysteresis in percent of the PV span can be set using the ALARM_HYS parameter. The priority of
each alarm is set in the following parameters:

HI_PRI

HI_HI_PRI

LO_PRI

LO_LO_PRI
Table D-3. Alarm Priority Levels
Number Description
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 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.