Rockwell Automation 1769-IT6 Compact I/O 1769-IT6 Thermocouple/mV Input Module - Manual (Page 36)

36 Rockwell Automation Publication 1769-UM004B-EN-P - March 2010
Chapter 3 Installation and Wiring
Cold Junction
Compensation To obtain accurate readings from each of the channels, the cold junction
temperature (temperature at the module’s terminal junction between the
thermocouple wire and the input channel) must be compensated for. Two cold
junction compensating thermistors have been integrated in the removable
terminal block. These thermistors must remain installed to retain accuracy.
If either of the thermistor assemblies are accidentally removed, re-install them by
connecting each one across each pair of CJC terminals.
Calibration The thermocouple module is initially calibrated at the factory. The module also
has an autocalibration function.
When an autocalibration cycle takes place, the module’s multiplexer is set to
system ground potential and an A/D reading is taken. The A/D converter then
sets its internal input to the module’s precision voltage source, and another
reading is taken. The A/D converter uses these numbers to compensate for
system offset (zero) and gain (span) errors.
Autocalibration of a channel occurs whenever a channel is enabled. You can also
program your module to perform cyclic calibration cycles, every five minutes.
See Selecting Enable/Disable Cyclic Calibration (word 6, bit 0) on page 50 .
To maintain optimal system accuracy, periodically perform an autocalibration
cycle.
ATTENTION: Do not remove or loosen the cold junction compensating
thermistor assemblies located on between the two upper and lower CJC
terminals. Both thermistor assemblies are critical to be sure of accurate
thermocouple input readings at each channel. The module will operate in
the Thermocouple mode, but at reduced accuracy if either CJC sensor is
removed.
See Determining Open-circuit Response (bits 6 and 5) on
page 46 .
IMPORTANT The module does not convert input data while the calibration cycle is in
progress following a change in configuration. Module scan times are
increased by up to 112 ms during cyclic autocalibration.