Fail-Safe Circuits - Omron NX-PA Series User Manual

4 Installation and Wiring
4-2

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, other Units, or slaves or due to other external factors affecting
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 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 all outputs from Digital Output Units in the following cases. The
slaves will operate according to the settings in the slaves.
• If a power supply error occurs.
• If a CPU error or CPU reset occurs.
• If a Controller error in the major fault level occurs.
• While the CPU Unit is in startup status until RUN mode is entered after the power is turned ON.
• If a system initialization error occurs.
External safety measures must be provided to ensure safe operation of the system in such
cases.
• If external power supplies for slaves or other devices are overloaded or short-circuited, the volt-
age will drop, outputs will turn OFF, and the system may be unable to read inputs. Provide exter-
nal 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 or memory for
CJ-series Units. As a countermeasure for such problems, external safety measures must be pro-
vided 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.
• 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.
• 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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WARNING
NX-series CPU Unit Hardware User's Manual (W535)