Rockwell Automation 1746-NT8 SLC 500 Thermocouple/mV Analog Input Module User Manual - Manual (Page 49)

Publication 1746-UM022B-EN-P - January 2005
Channel Configuration, Data, and Status 4-9
Select Open-Circuit State (Bits 7 and 8)
The open-circuit bit field lets you define the state of the channel data
word when an open-circuit condition is detected for that channel. This
feature can be disabled by selecting the disable option.
An open-circuit condition occurs when the thermocouple itself or its
extension wire is physically separated or open. This can happen if the
wire gets cut or disconnected from terminal block.
If either of the two CJC devices is removed from the terminal block,
any input channel configured for either a thermocouple or CJC
temperature input is placed in an open-circuit condition. An input
channel configured for millivolt input is not affected by CJC open
circuit conditions.
The results of the data word in an open-circuit condition depend
upon the selection of bits 7 and 8.
If zero is selected (00), the channel data word is forced to 0 during an
open-circuit condition.
Selecting maximum forces the (01) channel data word value to its
full scale value during an open-circuit condition. The full scale value
is determined by the selected input type and data format.
Selecting minimum forces the (10) channel data word value to its
low scale value during an open-circuit condition. The low scale value
is determined by the selected input type and data format.
Disabling the open-circuit selection (11) may result in unintended
operation on a failure. Generally, with the open-circuit option
disabled, the data word remains unchanged. The open-circuit error bit
and the channel LED flags the condition until the error is resolved.
IMPORTANT
Data resolution is not equivalent to data accuracy.
Input accuracy of ±50 µV may span multiple steps
for PID and Proportional Counts data types. As an
example, a Type B thermocouple temperature range
of 0 to 1820°C provides a voltage input range of 0 to
13.82mV to the 1746-NT8. This is a very small input
range and, when it is scaled to PID or proportional
counts ranges, a small input change results in many
counts being changed.