Allen-Bradley SLC 500 1746-NR4, User Manual

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Chapter 3
Installation and Wiring
3–13
The accuracy of a system that uses the RTD module is determined by the
following:
•
the accuracy of the RTD
•
resistance mismatch of the cable wires that connect the RTD to the
module
•
the accuracy of the RTD module
For optimal performance at the customer site, the RTD module is calibrated
at the factory prior to shipment. In addition, a self–calibration feature, called
autocalibration, further ensures that the module performs to specification
over the life of the product.
Factory Calibration
The 4–pin calibration connector, on the RTD module circuit board, is used
for factory setup only.
Autocalibration
When a channel becomes enabled, the module configures the channel and
performs an autocalibration on the channel. The channel is selected, the
excitation current is turned off, and the three input lines for the channel are
connected to analog common. The module’s A/D converters are configured
for the proper gain and filter frequency that is appropriate for your RTD
configuration. Autocalibration performs an A/D conversion on the zero
voltage (analog common) and the full–scale voltage (A/D reference voltage)
on the following signals:
•
lead wire signal
•
RTD/resistance signal
•
excitation current signal
Important: Channel calibration time is shown in Table 4.C.
These conversions generate offset (zero reference) and full scale (span
reference) coefficients that are saved and used by the module to perform
future A/D conversions on this channel.
You can command your module to perform an autocalibration cycle by
disabling a channel, waiting for the channel status bit to change state (1 to 0)
and then re–enabling that channel. Several scan cycles are required to
perform an autocalibration (refer to page xii). It is important to remember
that during autocalibration the module is not converting input data.
Calibration