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Code 15120: If a power failure occurs now, th e last data change d will be lost; index/buffer size = %1

Issue Description & Safety Warning

The Siemens SINUMERIK 840D CNC has triggered a critical If a power failure occurs now, th e last data change d will be lost; index/buffer size = %1 alarm. Immediate attention from a qualified operator or maintenance technician is required to prevent secondary damage to the equipment or spoiled parts during the manufacturing cycle. Always ensure standard safety and LOTO procedures are followed before accessing electrical cabinets.

Official Troubleshooting Steps

Parameters: %1 = Index/buffer size Definitions: Notification alarm. The alarm has no negative impact on the current machining. One of the system-internal data buffers, in which the last changed, buffered data are stored, has overflowed because the current data change rate is too high. The alarm warns that a spontaneous power failure in this situation (mains fault, disconnect the system from the power supply) would cause a loss of the immediatel y previously changed buffered data (tool data, parts programs, R parameters, GUDs,...) If the system is operated in an environment in which a power failure cannot occur, then the output of this alarm can be prevented via machine data MD18232 $MN_MM_ACTFILESYS_LOG_FILE_MEM[ index ] = 0. For information, parameter %1 specifies the index of the machine data, and the buffer size set there. Reaction: Alarm display. Remedy: If the alarm is present only sporadically, it can simply be regarded as a notification. The regular control behavior is not affected. If one does not want to or cannot eliminate the cause, then the alarm can be suppressed by setting MD11415 $MN_SUPPRESS_ALARM_MASK_2; Bit3=1 ('H8'). If the alarm is permanently present, please info rm the authorized personnel/service department. The value of MD18232 $MN_MM_ACTFILESYS_LOG_FILE_MEM[ index ] will then have to be increased. Program Continuation: Alarm display showing cause of alarm disapp ears. No further operator action necessary.

Source: SINUMERIK 840D sl / 840Di sl / SINAMICS S120 Diagnostics Manual (DAsl, 03/2009)

Last reviewed: 2026-07-01 · Verified against the manufacturer’s official documentation.

About Siemens SINUMERIK 840D CNC Alarms

Siemens controllers include extensive self-diagnostics that flag or halt the machine when a fault is detected. Error code 15120 on the SINUMERIK 840D CNC identifies one specific fault condition — its documented cause and the official corrective steps are listed above, sourced from Siemens's service documentation. Always confirm against the manual for your exact machine configuration and follow LOTO procedures before servicing.

Complete Alarm List: 4014 Siemens SINUMERIK 840D CNC codes.

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