Fail-Safe Circuits - Omron NX 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 ab-
normality 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 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.
• 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.
a) If a power supply error occurs.
b) If a CPU error or CPU reset occurs.
c) If a Controller error in the major fault level occurs.
d) While the CPU Unit is in startup status until RUN mode is entered after the pow-
er is turned ON.
e) 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 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 or
memory for CJ-series Units. As a countermeasure for such problems, external safe-
ty 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.
• 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.
• 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.
4-2-1 Order of Powering Up the Controller and Controlled System
Outputs from Units, such as DC Output Units, may malfunction momentarily when the power to the
Controller is turned ON. This may cause problems in the system if the Controller power supply is
turned ON after the controlled system's power supply. To prevent possible malfunctions, add an exter-
nal circuit that prevents the power supply to the controlled system from going ON before the power
supply to the Controller itself.
4-6
WARNING
NX-series CPU Unit Hardware User's Manual (W535)