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Field Calibration
Before performing a point field calibration, ultra stable, known,
precise values are connected to the input channel. During
point calibration, the ASI module reads the input and stores its
digitized value. Two point calibrations within an input range
must be performed. All these adjustment factors are calculated
and stored in the ASI module’s nonvolatile memory when the
point is enabled.
Precision voltage values are required for low level, thermocou-
ple, and high level input field calibration. For RTD input cali-
bration, precision resistance values are used. FC 217 and the
same procedure is used for all three field calibrations.
Each input channel must be enabled before normal input scan
processing is resumed. The enable command indicates to the
ASI module that the calibration process is completed. At that
time the ASI module stores the field calibration values in non-
volatile memory. The module uses these values during input
scan processing to calculate the correction factors for the cali-
brated channel.
If no field calibration is performed, or if the escape command
was issued to cancel the calibration, the new calibration cor-
rection values are not computed and the previous correction
factors will continue to be used. There is no change to the cali-
bration of the point. The quality of the input point remains bad
after an enable command until a successful input scan is per-
formed for that point. If either the controller or the ASI module
is stopped, or placed into configure mode while a field calibra-
tion sequence is in progress (before an enable is issued), the
field calibration sequence is aborted and must be restarted.
Once the ASI module is calibrated, no further manual calibra-
tion is necessary. Onboard, low-drift references maintain the
accuracy of the calibration.
Procedure
The original ASI module calibration is performed in the factory
in a controlled, stable environment. Ambient temperature
must remain constant at 25 (±1.0) degrees C. The field calibra-
tion location must be free from drafts.
After each ASI module is powered-up in its calibration location,
allow some warmup time before calibration for the most accu-
rate results. This warmup time should be at least 15 minutes.
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