Rockwell Automation T80004 Application Note Field Loop Configuration - Manual (Page 15)

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AN-T80004 Field Loop Configuration
Issue 10 Jun 13 AN-T80004 15
Overvoltage Faults
The analogue inputs are not designed to measure voltages up to 24V. The A/D conversion will reach
its maximum at about 7 volts. Above this point, the measurement parameters will be at the maximum
values. This causes module channel faults (0x52nn) to be reported, which appear as slice faults.
Voltage signals to Analogue Input Modules
This section describes the issues involved with the connection of voltage signals to T8431 analogue
input modules without the use of a 250 ohm burden resistor, usually used to convert current loops to
voltage signals.
Background: Input impedance test
Current loop analogue signals are wired to a 250 ohm burden resistor to convert 0-20mA to 0-5V (as
an example). The input circuits are biased internally to 2.5V. An input impedance test is run periodically
inside the module. This test adds a disturbance signal to the input circuit which is later filtered out
before the measurement is delivered to the processor. The test measures the effect of the disturbance
and calculates the impedance of the input circuit. The 250 ohm burden resistor is factored out of the
calculation, to leave the true impedance of the line. Out of range (short circuit or open circuit)
impedances are recorded. If an out-of-range impedance appears on several subsequent tests on at
least two input circuit slices, then a fault state is declared on the channel.
With a voltage input which has very low or high impedance, the above test will declare a fault after
confirmation and voting, which can take a few minutes. The result is that the input state number is set
to the fault state and the input measurement is set to -2048 as the failsafe value.
Workaround
The recommended method to enable the use of voltage inputs is to fit a 250 ohm resistor. The value is
nominal in this case (e.g. 200-300 ohms) and simply biases the impedance into the accepted test
range. For low impedance inputs, the resistor should be wired in series to add impedance to the line.
For high impedance inputs, the resistor should be wired in parallel as in a current loop circuit. In both
cases, the impedance test should be able to detect line faults.
Correcting the input impedance will also improve the input measurement accuracy at the high and low
ends of scale. An offset will be observed at the ends of the range if the input impedance differs
significantly from 250 ohms.
MTEST Diagnostics
Applications have been reported that inhibit the module diagnostic tests by loading an ‘Additional CLI’
template with ‘MTEST=OFF’. This is not a recommended workaround. This command will turn off other
diagnostic functions in the module in addition to the impedance test. It is not stored as part of the INI
configuration in the module. This means that although it will be downloaded from the processor on
startup or on offline insertion of a module (where the active module is removed and a new module is
inserted), it is not transferred on a hot swap to a module in the secondary module position. Therefore if
a hot swap is performed to the secondary position, and the secondary module is left in operation, the
impedance test will shut down the inputs after a few minutes.
In these applications, it is necessary to swap back to the primary slot before the diagnostics have run,
or to remove and reinsert the module in the primary position. Both of these actions will reload the INI
from the processor in full. Removal and reinsertion requires redundancy logic in the application to
avoid loss of signal.