Rockwell Automation 1756-OF8I ControlLogix Eight-channel Isolated Analog I/O Modules - Manual (Page 170)

170 Rockwell Automation Publication 1756-UM540A-EN-P - May 2014
Chapter 9 Troubleshoot Your Module
DC signal on top of the input signal Offset readings can be caused by a DC signal riding on top of the input
signal. An oscilloscope shows the magnitude of the offset. Disconnect all but one input to see if channels are affecting each
other, that is, there is bleed-over.
Module is in calibration mode Incorrect readings can be a result of the module being calibrated
when the reading occurs. Make sure the module is not in calibration mode.
This symptom is module dependent, but in general, specific bits have
to be turned on to enable calibration.
Incorrect reading soon after module
installation One of the following:
• Incorrect readings at initial module installation and configuration
are often the result of installation and configuration errors.
• Incorrect readings at installation of an existing, previously-
working module are more likely the result of a hardware failure.
One of the following:
• If error is occurring on a new module installation and
configuration, make sure that those tasks were completed
corrected and correct any errors you find.
• If the error is occurring on an existing, previously-working
module, diagnose the hardware failure and correct it. If more than
one channel is experiencing the incorrect readings in this case,
disconnect all inputs except one. This can help determine if it is
external hardware or the module itself is the cause.
One of the following:
• Short-circuit condition -
Typically, caused by a short
between terminal. With this
module the short is between
terminals IN_x/I/SRC and RTN_x
(where x is the channel number).
• Sourcing overcurrent condition -
Caused by a current signal
>24 mA.
When a short-circuit or sourcing overcurrent condition exists, the
module sets the input to 24 mA, that is, the equivalent engineering
unit value. This value indicates a special error condition beyond the
normal Overrange value, that is, 21 mA:
The following events occur:
1. For one second, the short-circuit or overcurrent condition self-
corrects if the condition trigger is removed.
2. After one second, the condition latches, the channel disables
Source Loop.
3. Current and continue to send 24 mA with an Overrange indication.
Check wiring and correct terminations.
To unlatch the condition after the conditioning trigger is removed,
perform one of the following:
• Cycle power to the module.
• Reset the module.
• Inhibit and uninhibit the module.
Sensor Offset The software configured Sensor Offset value is summed with the
input data value Insure that the Sensor Offset feature is desired.
Programming and Configuration One of the following:
• The input value is not read from correct data tag.
• The channel is not configured for correct range.
• The Signal and Engineering units are not set correctly.
If the incorrect input data value is in a secondary location, for
example, an HMI device, verify the base tag value is in the controller.
Correct programming or configuration as needed.
Table 18 - 1756-IF8I Module - Troubleshoot Incorrect Input Voltage/Current Readings
Possible Cause of Incorrect
Reading Description Recommended Action