Normally, the status of the measurement value provided by the analog input card reflects the operating condition of the I/O card, and processing condition.
If you configure the block to use a HART Analog Input Channel, you can select which conditions associated with a HART channel impact the status of the block.
The channel input status to the analog input block is set high- or low-limited by the input card if the channel value exceeds the overrange and underrange limits and if the IO_IN parameter is selected as HART_FIELD_VAL or FIELD_VAL_PCT. You may change the default overrange and underrange limits independently on each input channel.
For classic I/O if the channel value is outside the range of -20.12 to 116.6%, the channel input status is set to BAD by the input card.
For CHARM I/O channel status depends on the configured CHARM functionality:
For 4 - 20 mA CHARMs, the card sets the channel status to BAD when the channel value is greater than 131.25% (25 mA).
For 0 - 20 mA CHARMs, the card sets the channel status to BAD when the channel value is greater than 125% (25 mA).
The STATUS_OPTS parameter allows you to choose how the PV and OUT status are determined. There are three AI block status options:
Uncertain if Man mode -- Sets the OUT status to Uncertain if the block's mode is Manual
Bad if Limited -- Sets the PV status to Bad if the channel input status is limited.
Uncertain if Limited -- Sets the PV status to Uncertain if the channel input status is limited.
You can set the status option in Out of Service mode only.
The analog input block PV status normally reflects the channel input status. However, if the channel status is limited, then PV status is set to Uncertain limited or Bad Limited if you select the status option Uncertain if Limited and Bad if limited respectively
In Auto mode, OUT normally reflects the value and status quality of the PV. However, if the PV status is Good but the PV value is more than -10% to 110% outside the span defined by PV_SCALE, then the OUT status is change to Uncertain.
In Man mode, the OUT status limit indication is set to constant and the OUT status is Good unless you have selected the Uncertain in Man mode status option.
In Out of Service mode, the OUT status is set to BAD.
NAMUR limit detection
If NAMUR limit detection is enabled for I/O channels (by setting the NAMUR_ENA parameter to True when configuring the AI card) and the IO_IN type is FIELD_VAL_PCT or HART_FIELD_VAL, the PV status of the input is set to Bad if the signal level is above 21 mA or below 3.6 mA for more than four seconds. The Bad status is cleared when the signal returns within these limits. You can use this feature when the transmitter is designed to flag a device failure by setting its current signal outside the normal 4-20 mA range.
The actual mode of a control block will automatically go to Man if the input has a Bad status. Thus, NAMUR limit detection should not be enabled if a measurement is used in control unless this behavior (control going to manual ) is an appropriate action on limit detection.