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5 Installation and Wiring
NX-series NX1P2 CPU Unit Hardware User’s Manual (W578)
5-2 Fail-safe Circuits
5
5-2 Fail-safe Circuits
Provide safety measures in external circuits to ensure safety in the system if an abnormality occurs
due to malfunction of the CPU Unit, slaves, or Units or due to other external factors affecting oper-
ation. 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 pro-
vided in external control circuits.
• The outputs may remain ON or OFF due to deposition or burning of the output relays or destruc-
tion of the output transistors. As a countermeasure for such problems, external safety measures
must be provided to ensure safe operation of the system.
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.
• If a communications interference occurs for remote I/O or if a major fault level error occurs, out-
put 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 external 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 counter-
measure 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 safety in the over-
all system even if errors or malfunctions occur in data link communications or remote I/O commu-
nications.
• 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 interruptions. Not doing so
may result in serious accidents due to incorrect operation.
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