Code 120203: Communication failure
Issue Description & Safety Warning
The Siemens SINUMERIK 840D CNC has triggered a critical Communication failure 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
Definitions: The operator panel is connected to the NC and PLC via a serial bus. This alarm occurs if the MMC is started for the first time and the NC/PLC ramp-up is not yet finished or the communication to these components is faulty. In connection with this alarm, all display va lues connected with the NC/PLC become invalid. Such faults are normal while the contro ls are ramping up (e.g. after resetting). Reaction: Alarm display. Remedy: The alarm disappears automatically as soon as the fault situation has ended. If this alarm is continuously present, a wide variety of faults may be the cause. (e.g. wire breakage, NC/PLC not ramped up, faulty address/data transfer rate configuration of one of the bus nodes, etc.). Program Continuation: Internal
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 120203 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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