Allen-Bradley 1444-TSCX02-02RB User Manual page 259

Operate the Module Chapter 10
When a sensor fails, the external circuit can go to an open condition. It can also
be driven towards zero or the provided source power voltage (typically ±24V),
depending on the nature of the fault. For an open condition, the circuit design
forces the bias to move rapidly to a 'fault' state. How quickly the bias level
transitions to its fault state depends on the specified failure mode, the bias level
at the time of the fault, and the bias limits entered. It is not possible to verify
that for every possible fault mode, limit selection, and signal conditioning
solution that the bias transitions past its fault limits within a known time.
Consequently it is recommended that alarms be defined with not less than 1
second delay. This delay makes sure that if a transducer fault occurs, then the
fault is detected before the alarm is enunciated.
Current Based Fault Detection
For negatively powered eddy current probes (only), the Dynamix 1444 Series
includes dedicated hardware to monitor the current being provided to the
probe driver and the bias level returned. This feature provides fast detection of
supply current that drops below 2 mA or a positive bias voltage being detected.
Either or both detections trigger a 'wire-off ' state to be declared which is
normally then an input to the TX OK state.
Clearing a Fault
When monitoring a wire-off condition for powered eddy current probes,
measurements that are made on the associated channel can spike as they
transition to their normal state. For example, when a faulted sensor is replaced
or a loose wire is reconnected. As wires are connected and tightened in place,
momentarily intermittent (on/off ) connections often exacerbate this
condition.
To help prevent further alarms due to these transitory events, the module
latches any 'wire-off ' state for 30 seconds after the fault condition has cleared
before transitioning a faulted transducer to its normal, non-faulted state.
Speed Inputs
Two inputs are provided to accept transistor-transistor logic (TTL) signals.
TTL signals need a clear distinction between 'low' and 'high' values such that a
trigger threshold of 2.5V does not falsely trigger on high or low signal level or
noise.
These inputs are designed to sample sufficiently fast to satisfy the module
speed measurement specifications.
Rockwell Automation Publication 1444-UM001D-EN-P - June 2018 259