Siemens SIMATIC S7-1200 System Manual page 1311

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S7-1200 behavior following a fatal error

If the CPU firmware detects a fatal error it attempts a defect-mode restart, and if successful,

signals the defective mode by continually flashing the STOP/RUN, ERROR and MAINT

LEDs. The user program and hardware configuration are not loaded following the defect-

mode restart.

If the CPU successfully completes the defect-mode restart, the CPU and signal board

outputs are set to 0, and the outputs of central rack signal modules and distributed I/O are

set to the configured "Reaction to CPU STOP".

If the defect-mode restart fails, (for example, due to a hardware fault), the STOP and

ERROR LEDs are ON and the MAINT LED is OFF.

Operation in defect state cannot be guaranteed

Control devices can fail in an unsafe condition, resulting in unexpected operation of

controlled equipment. Such unexpected operations could result in death or serious injury to

personnel, and/or damage to equipment.

Use an emergency stop function, electromechanical overrides or other redundant

safeguards that are independent of the PLC.

Status LEDs on an SM

In addition, each digital SM provides a DIAG LED that indicates the status of the module:

● Green indicates that the module is operational

● Red indicates that the module is defective or non-operational

Each analog SM provides an I/O Channel LED for each of the analog inputs and outputs.

● Green indicates that the channel has been configured and is active

● Red indicates an error condition of the individual analog input or output

In addition, each analog SM provides a DIAG LED that indicates the status of the module:

● Green indicates that the module is operational

● Red indicates that the module is defective or non-operational

The SM detects the presence or absence of power to the module (field-side power, if

S7-1200 Programmable controller

System Manual, V4.2, 09/2016, A5E02486680-AK

Online and diagnostic tools

15.1 Status LEDs