Spectrum Controls 1769sc-IT6I - Manual (Page 40)

4-6 Compact™ IO Isolated Thermocouple Module
User’s Manual Pub. 0300244-01 Rev. A
4.2.7 Determining Open-Circuit Response (Bits 6 and 5)
An open-circuit condition occurs when an input device or its extension wire is physically
separated or open. This can happen if the wire is cut or disconnected from the terminal
block.
Note: If either CJC sensor is removed from the module terminal block, its open-circuit
bit is set (1) and the module continues to calculate thermocouple readings at reduced
accuracy. If an open CJC circuit is detected at power-up, the module uses 25°C as the
sensed temperature at that location. If an open CJC circuit is detected during normal
operation, the last valid CJC reading is used. An input channel configured for
millivolt input is not affected by CJC open-circuit conditions. See Open-Circuit
Detection on page 5-3 for additional details.
Bits 6 and 5 define the state of the channel data word when an open-circuit condition is
detected for the corresponding channel. The module overrides the actual input data
depending on the option that you specify when it detects an open circuit. The open-
circuit options are explained in the table below.
Table 4-4 (Open-Circuit Response)
Response
Option
Definition
Upscale Sets the input data value to full upper scale value of channel data word.
The full-scale value is determined by the selected input type and data
format.
Downscale Sets the input data value to full lower scale value of channel data word.
The low scale value is determined by the selected input type and data
format.
Last State Sets the input data value to the last input value prior to the detection of
the open-circuit.
Zero Sets the input data value to 0 to force the channel data word to 0.
4.2.8 Display CJC Data (Bit 4)
When Bit 4 is set, the CJC compensated temperature is displayed for each channel
instead of the measured thermocouple or millivolt reading. Setting bit 4 to 0 allows the
measured thermocouple or millivolt reading to be displayed.
4.2.9 Selecting Input Filter Frequency (Bits 0 through 2)
The input filter selection field allows you to select the filter frequency for each channel 0
through 5. The filter frequency affects the following, as explained later in this chapter:
•
Noise rejection characteristics for module inputs
•
Cut-Off Frequency
•
Repeatability
•
Module update time
The table below summarizes the effects of each filter selection.