DESCRIPTION AND OPERATION
AUTOMATIC ADJUSTMENTS AND CORRECTIONS
I-E96-301B
2 - 7
Input Calibration
Each input channel is calibrated at the factory. During the cal-
ibration procedure, any offset, gain and nonlinearity errors are
identified and required correction factors are calculated and
stored in the slave's nonvolatile memory.
These factors are used when the input channel is scanned to
correct the reading. A calibration adjustment is applied to
active channels and the cold junction reference input.
Periodic reference readings are taken to provide a correction for
drift of input channel components. This drift calibration is per-
formed automatically and continually while normal input pro-
cessing is maintained.
Lead Wire Resistance Adjustment
All but high level inputs are adjusted for lead wire resistance
coming into the termination device (The effect of lead wire
resistance on high level inputs is negligible). This value is sup-
plied through a function code specification and is used by the
slave module in its input value calculation. The adjustment for
lead wire resistance depends upon the input channel circuitry
used for a particular input signal type. For millivolt and ther-
mocouple inputs, lead wire resistance is equal to the series
resistance of the + and - leads. For 3-wire RTD inputs, the lead
wire resistance specified in function code 216 specification 10
is equal to the resistance of any ONE of the three leads. All
three resistances must be equal for accurate lead wire resis-
tance compensation.
The resistance of the cable from the termination module to the
slave module is also taken into account when making this
adjustment.
Cold Junction Compensation
Thermocouple input channels are adjusted for cold junction
temperature. The cold junction reference may be the built-in
reference available on the slave's termination device, or may be
a value originating from any other function code block output
anywhere in the INFI 90 system. The cold junction reference
supplied by the MFP is assumed to be in degrees C. The slave
converts this value to millivolts and adds it to the value from
the analog-to-digital converter.
Thermocouple channels identified to be cold junction reference
inputs (function code 216 specification 4) use the built-in cold
junction reference on the termination device for their cold
junction compensation.
