System and operating states of the S7–400H
11.3 The operating states of the CPUs
S7-400H
136 System Manual, 03/2012, A5E00267695-11
4. If a multiple-bit error occurs on a CPU in redundant mode, that CPU will enter ERROR-
SEARCH mode. The partner CPU assumes master mode as required, and continues
operation in standalone mode.
But: OB 84 is called if 2 single-bit errors occur on a CPU in redundant mode within 6
months. The CPU does not change to ERROR-SEARCH mode.
5. If synchronization is lost during redundant mode, the standby CPU changes to ERROR-
SEARCH mode. The other CPU remains master and continues operation in standalone
mode.
The purpose of ERROR-SEARCH mode is to find a faulty CPU. 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 state. 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 error-
search mode, the hardware of the previous master CPU is tested.
No communication is possible, e.g. by means of access from a programming device, while
the CPU is in ERROR-SEARCH operating state. The ERROR-SEARCH mode is indicated
by the RUN and STOP LEDs, see section Status and error displays (Page 55).
For ad
ditional information on
the self-test, refer to section Self-test (Page 137)
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