Rockwell Automation Publication 1769-UM021G-EN-P - October 2015 201
Use I/O Modules with CompactLogix 5370 L2 Controllers Chapter 8
Open Circuit Detection Update Time
Open circuit detection is used to verify that the field wiring is physically
connected to the embedded analog input point. If this feature is enabled and field
wiring is disconnected from the input, the application alerts you to the condition
and an open wire bit is set for the respective input channel in the tags for the
project.
Open circuit detection can be enabled or disabled on any channel input type
except for an input channel that is configured to use the 0…20 mA input range.
The configuration selection is made on the Input Configuration tab on the
Module Properties dialog box, as shown in the following graphic. The
configuration choice, that is, enabled or disabled, is the result of an Open Circuit
Response selection for the channel.
Keep the following in mind when calculating the CJC update time:
• If multiple input channels are configured to use a thermocouple input type
and another filter value is selected for each, the filter frequency selection
with the slowest update time determines the filter frequency selection
with the slowest update time determines the CJC update time.
For example, if one input channel uses a thermocouple input with a 50 Hz
filter frequency and another input channel uses a thermocouple input with
a 60 Hz filter frequency, the CJC channel update time is 134 ms.
• The CJC update time that increases overall module update time is only used
once regardless of the number of input channels on a module that have CJC
enabled to scan every other scan.
In other words, if your module uses a filter frequency selection of 250 Hz
and includes three channels with CJC enabled to scan every other scan, you
only add one instance of CJC update time to the overall equation. Instead of
including 38 ms for each channel, you include 38 ms once.
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