Chapter 2
Description
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Rockwell Automation Publication ICSTT-RM451D-EN-P - March 2021
The resistors in series with the gate of Switch 1 and the signal transistor serve
to protect the drive logic if there is a malicious switch failure. The pull-up
resistors define the gate voltages in the absence of power.
Switch Diagnostics
During normal operation, Switch 1 and Switch 2 are maintained on. In this
state, Switch 1 and Switch 2 exhibit a low resistance.
To determine the ability of the system to control the load via Switch 1 and
Switch 2, their gate voltages are modulated, one at a time. As the gate voltages
are modulated, the monitoring signals synchronously change in a predictable
fashion. The local DSP analyzes the relative amplitude and phase of these
small AC signals, to determine the on resistance and threshold voltages of
each switch.
The current to the load does not need to be completely interrupted to obtain a
level of confidence in the ability of the transistors to turn off. For the TMR
switch configuration in the on state, only one fail-safe switch at a time needs
to be modulated, while the other two bear the load current.
Short Circuit Protection
Output channels are classified as protected under IEC 61131-2, specifically
‘Protected Outputs Requirements’.
While energizes, the channel output current is monitored to protect the
channel switches from damage by a sustained overcurrent condition. Having
detected a sustained overcurrent condition a short-circuit fault state is
latched and the output is de-energized.
While de-energized, the output may be optionally be configured to detect
short-circuit field faults (resistance less than 40 Ω) through System .INI file
configuration. The detection of short-circuits while de-energized relies upon a
partial energizing of the field circuit to determine the channel load resistance.
The partial energizing of a field circuit is designed to constrain any change in
field voltage or current to less than 2V or 100 mA, respectively.
A latched short-circuit fault state can be cleared by operating the system Fault
Reset button or by transitioning the commanded channel state.
The output also includes a non-replaceable fusible link for absolute protection
Group Fail-Safe Switches
To support safe operation, the Output Module is equipped with a series of
switches that provide source power to a group of eight output channels, a
Power Group. The output Module Group Fail-Safe Switch (GFSS) is intended
