Spectrum Controls 1746sc-NI8u - Manual (Page 55)

Chapter 4: Channel Configuration, Data, and Status 43
Important: Data resolution is not equivalent to data accuracy. Data
resolution merely indicates what a bit-weight is in any given
input type and data format combination. Input accuracy of
±50µV 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 NI8u. This is a very small
input range and when it is scaled to PID or proportional
counts ranges a small input change will result in many
counts being changed.
Select Open Circuit State (Bits 8 and 9)
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. The open
circuit does not apply to the 0-5V, 1-5V, 0-10V, ±2V, ±10V, or 0-20mA
input types and should be disabled when those types are selected, or else a
configuration error will result. It can be enabled for all other types,
including the CJC input. This feature can be disabled by selecting the
disable option.
An open circuit condition occurs when the input path is physically
separated or open. For thermocouples or RTDs, either the sensor or the
extension wire may be broken. The voltage or current input wire may be
cut or disconnected from the terminal block. For RTDs only, a short
circuit of less than 3 ohms will also flag this error.
If either of the two CJC devices are removed from the module wiring
terminal, any input channel configured for either a thermocouple or CJC
temperature input will be placed in an open circuit condition. An input
channel configured for millivolt, volt, milliamp, or RTD 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 8 and 9.
If zero is selected, the channel data word is forced to 0 during an open-
circuit condition.
Selecting maximum forces the 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 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.
When the open-circuit option applies, disabling the open-circuit selection
may result in unintended operation on a failure because the returned data
word value is unknown. The open circuit error bit and the channel LED
will flag the condition until the error is resolved.