Siemens SIMATIC S7-400H System Manual page 88

System and operating states of the S7-400H
8.4 The operating states of the CPUs
4. When a multiple-bit error occurs on only one of the redundant CPUs, that CPU will enter
5. If synchronization is lost during redundant mode, the standby CPU changes to
The TROUBLESHOOTING MODE is set to allow a faulty CPU to be localized. The standby
CPU runs the full self-test, while the master CPU remains in RUN.
If a hardware fault is detected, the CPU changes to DEFECTIVE mode. If no fault is detected
the CPU is linked up again. The fault-tolerant system resumes the redundant system state.
An automatic master-standby changeover then takes place. This ensures that when the next
error is detected in troubleshooting mode, the hardware of the previous master CPU is tested.
No communication is possible with the CPU in TROUBLESHOOTING mode, for example no
access by a programming device. The TROUBLESHOOTING mode is indicated by the RUN
and STOP LEDs; see section Status and error displays (Page 46).
For further information on the self-test, refer to section Self-test (Page 89)
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the TROUBLESHOOTING state. The partner CPU assumes master mode as required,
and continues operation in single mode.
But: OB 84 is called when a single-bit error occurs on one of the redundant CPUs.
The CPU does not change to TROUBLESHOOTING mode.
TROUBLESHOOTING mode. The other CPU remains master and continues operation in
single mode.
System Manual, 09/2007, A5E00267695-03
S7-400H