Rockwell Automation 1746-NR8 SLC 500 RTD/Resistance Input Module User Manual - Manual (Page 35)

Publication 1746-UM003A-EN-P
Installation and Wiring 2-15
Calibration 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, an autocalibration feature further
ensures that the module performs to specification over the life of the product.
Factory Calibration
The 2-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 module if the combination of input type
and excitation current are unique to that channel. Autocalibration performs
A/D conversions on the zero voltage and the full-scale voltage of the A/D
converter.
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.
You can command your module to perform an autocalibration cycle once every
5 minutes by setting any channel’s calibration disable bit to 0. To disable
autocalibration, all channel’s calibration disable bits must be set to 1. You can
control the module’s autocalibration time by disabling autocalibration, and
then setting any channel’s calibration disable bit to 0, waiting at least one
module scan time and then resetting that channel’s calibration disable bit to 1.
Several scan cycles are required to perform an autocalibration (see page 3-10).
It is important to remember that during autocalibration the module is not
converting input data.
IMPORTANT
Channel calibration time is shown in “Channel
Autocalibration” on page 3-10.