Rockwell Automation 1769-IT6 Compact I/O 1769-IT6 Thermocouple/mV Input Module - Manual (Page 46)

46 Rockwell Automation Publication 1769-UM004B-EN-P - March 2010
Chapter 4 Module Data, Status, and Channel Configuration
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.
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 th
at you specify when it detects an
open circuit. The open-circuit options are explained in this table.
Selecting Input Filter Frequency (bits 2…0)
The input filter selection field allows you to select the filter frequency for each
channel and provides system status of the input filter setting for channels 0…5.
The filter frequency affects the following, as explained later in this chapter:
• Noise rejection characteristics for module inputs
• Channel step response
• Channel cut-off frequency
• Effective resolution
• Module update time
Effects of Filter Frequency on Noise Rejection
The filter frequency that you choose for a module channel determines the
amount of noise rejection for the inputs. A lower frequency (50 Hz versus
500 Hz) provides better noise rejection and increases effective resolution, but
also increases channel update time. A higher filter frequency provides lower noise
rejection, but decreases the channel update time and effective resolution.
TIP 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 powerup, the module uses 25 °C (77 °F) 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 77 for additional details.
Table 3 - Open-circuit Response Definitions
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.