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77GFK-0827 Chapter 4 Operation
Fault Actions in a CPU Redundancy System
Fault actions in the Hot Standby CPU Redundancy System are handled differently than
those in a non-redundancy (Simplex) system. Whenever there is a ”ready” backup unit
configured in the system, the fault actions in the active unit will not be those normally
specified by the user. For an active unit with a ready backup, any fault which will cause
a degradation in performance, or loss of control of I/O which does not cause a similar
degradation of performance or loss of control of I/O in the backup will be considered a
FATAL fault in the active unit and cause the active unit to transition to STOP mode.
The configurable fault actions will be applied whenever the system is running in
stand-alone mode in case you prefer fault tolerance (availability) versus safety
(depending on the application). The following tables (1) define the Fault Groups and (2)
show the fault groups and their fault action defaults. There are three fault actions
shown in Table 11: Fatal, Non-Fatal, and Conditionally Fatal. Fatal always stops the PLC,
Non-Fatal never stops the PLC and Conditionally Fatal stops the PLC depending on
other information in the fault.
Table 9. Maskable Fault Group Descriptions
Fault Group
Table
Type Description
LOSS_RACK CPU Loss of or Missing Rack
LOSS_IOC I/O Loss of or Missing IOC
LOSS_IO_MOD I/O Loss of or Missing I/O Module
LOSS_OTHR_MOD CPU Loss of or Missing Option Module
ADD_RCK CPU Addition of or Extra Rack
ADD_IOC I/O Addition of or Extra IOC
ADD_IO_MOD I/O Addition of or Extra I/O Module
ADD_OTHR_MOD CPU Addition of, Reset of, or Extra Option Module
IOC_FAULT I/O IOC or I/O Bus Fault
IO_MOD_FAULT I/O I/O Module Fault
CNFG_MIS_MTCH Both System Configuration Mismatch
SYS_BUS_ERROR CPU System Bus Error
CPU_HARDWR CPU CPU Hardware Failure
MOD_HARDWR CPU Module Hardware Failure (e.g. Serial Port Failure on PCM)
IOC_SOFTWR I/O IOC Software Failure
MOD_OTHR SOFTWR CPU Option Module Software Failure
PRG_BLK_CHKSUM CPU Program Block Checksum Mismatch
LOW_BATTERY CPU Low Battery in the System
CNST_SW_EXCD CPU Constant Sweep Exceeded
PLC_FTBL_FULL CPU PLC System Fault Table Full
IO_FTBL_FULL CPU I/O Fault Table Full
APPLICATION_FLT CPU User Application Fault