5-2
Fail-safe Circuits
WARNING
Provide safety measures in external circuits to ensure safety in the system if an ab-
normality occurs due to malfunction of the CPU Unit, slaves, or Units or due to other
external factors af
fecting operation. Not doing so may result in serious accidents due
to incorrect operation.
•
Emergency stop circuits, interlock circuits, limit circuits, and similar safety measures
must be provided in external control circuits.
• The outputs may remain ON or OFF due to deposition or burning of the output re-
lays or destruction of the output transistors. As a countermeasure for such prob-
lems, external safety measures must be provided to ensure safe operation of the
system.
WARNING
The CPU Unit will turn OFF digital outputs on the CPU Rack in the following cases.
• While the CPU Unit is on standby until RUN mode is entered after the power is
turned ON
•
If an error occurs in the power supply
• If a system initialization error occurs
Digital outputs on the CPU Rack will produce outputs according to the settings in the
following cases.
• If a CPU Unit error or CPU Unit reset occurs
• If a major fault level Controller error occurs
External safety measures must be provided to ensure safe operation of the system in
such cases.
WARNING
• If a communications interference occurs for remote I/O or if a major fault level error
occurs, output status will be determined by the specifications of the external output
device.
Confirm the operating specifications for communications errors and major fault level
errors and implement suitable safety measures.
Correctly set all of the settings in the slaves and Units.
• If external power supplies for Units, slaves or other devices are overloaded or short-
circuited, the voltage will drop, outputs will turn OFF
, and the system may be unable
to read inputs. Provide external safety measures in controls with monitoring of ex-
ternal power supply voltage as required so that the system operates safely in such
a case.
• Unintended outputs may be performed when an error occurs in variable memory. As
a countermeasure for such problems, external safety measures must be provided to
ensure safe operation of the system.
• Provide measures in the communications system and user program to ensure safe-
ty in the overall system even if errors or malfunctions occur in data link communica-
tions or remote I/O communications.
• You must implement fail-safe circuits to provide sufficient protection in the event that
abnormal signals occur as the result of broken signal lines or momentary power in-
terruptions.
Not doing so may result in serious accidents due to incorrect operation.
5 Installation and Wiring
5-8
NX-series NX1P2 CPU Unit Hardware User’s Manual (W578)
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