Rockwell Automation 1756-XXXX ControlLogix Analog I/O Modules - Manual (Page 131)

Publication 1756-UM009C-EN-P - December 2010 131
Temperature-measuring Analog Modules (1756-IR6I, 1756-IT6I, and 1756-IT6I2) Chapter 6
Wire Length Calculations
The rule for determining the maximum thermocouple lead length without
error is that the lead length’s error should be less than half of the module
resolution. This error implies no error is observed nor is recalibration
required.
The resolution for the 1756-IT6I and 1756-IT6I2 module, respectively, is:
-12…30 mV range = 0.7 uv/bit
-12…78 mV range = 1.4 uv/bit
Based on the schematic on page 137 , the module leakage per open-wire current
is the bias voltage/pull-up resistance = 0.44V / 20 M
Ω
= 22 nA. Hence, the
maximum thermocouple loop resistance is the sum of total loop resistance =
both leads.
Using this equation, for the -12…30 mV range the maximum lead resistance is
16
Ω
for a maximum of half resolution error (1/2*(0.7uv/bit) / 22 nA).
For the -12…78 mV range, the maximum lead resistance is 32
Ω
for a
maximum of half resolution error (1/2*(1.4uv/bit) / 22 nA).
For more information, refer to 1756-IT6I and 1756-IT6I2 Thermocouple
Lead Length Calculations, Knowledgebase Technical Note ID 59091.
Differences Between
the 1756-IT6I and
1756-IT6I2 Modules The 1756-IT6I and 1756-IT6I2 modules support grounded and ungrounded
thermocouples. However, in addition to offering access to two more
thermocouple types (D and TXK/XK [L]), the 1756-IT6I2 module offers the
following:
• A greater cold-junction compensation accuracy
• Improved module accuracy
See page 135 for details.
While the 1756-IT6I module can report cold junction temperature differences
between channels as high as 3 °C from the actual temperature, the
1756-IT6I2 module, because it has two cold-junction sensors (CJS), reduces
the potential cold-junction error from actual temperature to 0.3 °C.
It’s important that you check that the CJS is fitted locally or remotely, and
enabled accordingly in the module channel configuration. If the CJS is not
fitted or if the sensor wiring leads are incorrect (for example, swapped over at
the thermocouple cards input), there’s a possibility of a negative or positive
temperature fluctuation when the thermocouple sensor is warmed.