5
REDUNDANT SYSTEM FUNCTIONS
5.1 Basic Concept of Redundant System
5.1.5 Self Diagnostics Function
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5.1.5 Self Diagnostics Function
(1) Self Diagnostics Function
The self diagnostics function means that the redundant CPU checks for its own error,
in order to prevent malfunction as well as to perform preventive maintenance.
If an error occurs when the redundant system is powered on or while the redundant
CPU is running, the redundant CPU detects the error and displays it and performs
system switching, etc.
(2) Error Detection Processing
(a) Error detection processing
When detecting an error, the redundant CPU will perform the following process:
1) Turning the ERR.LED etc. on
2) Turning the special relays (SM0, SM1) on
3) Storing the error information (error code) into the special resistor (SD0)
When detecting multiple errors, the redundant CPU will store only the latest
error code into the SD0.
Use the special relay and special register in a program to make a
programmable controller or machine system interlock.
(b) Error history confirmation
The redundant CPU records the 16 latest error codes as error history.
The error history can be checked in the PLC diagnostics of GX Developer.
The error history is backed up by the battery even if the programmable controller
is powered off.
(3) Redundant CPU Operation upon an error detection
(a) Operation Mode when detecting an error
When detecting an error through the self diagnostics, the redundant CPU
operates in the following two ways:
1) Stop of redundant CPU operation
Upon an error detection, the redundant CPU stops the operation and performs
system switching.
The redundant CPU holds all outputs (Y) that have not been assigned to the
main base unit, while it processes the outputs (Y) that have been assigned to
the main base unit according to "Output Mode on Error Detection" of the PLC
parameters I/O assignment settings as shown below:
• Turns off all output of modules set to "Clear Output" (default).
(Device memory output (Y) is held.)
• Output of modules set to "Maintain output" is held. (Device memory output
(Y) is maintained.)
2) Continuation of redundant CPU operation
Upon an error detection, the redundant CPU executes all programs other than
the program (instruction) where the error occurred.