Code 28004: NCU link: NCU %1 of the link network is not on the bus
Issue Description & Safety Warning
The Siemens SINUMERIK 840D CNC has triggered a critical NCU link: NCU %1 of the link network is not on the bus 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 = NCU number Definitions: Error message of the NCU link module. When the NCU link was powered up, the local NCU (indicated by the alarm) detected that the NCU with the number in the alarm parameter was not on the bus although it should be connected ac cording to the MD settings. This fault in the link can have various causes: - Defective hardware. - The machine data which configure the NCU link are not identical on all NCUs. - An identical interpolator cycle ti me has not been selected on all NCUs. Reaction: NC not ready. Channel not ready. NC Start disable in this channel. Interface signals are set. Alarm display. NC Stop on alarm. Remedy: Check the machine data configuration and link hardware. Program Continuation: Switch control OFF - ON.
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 28004 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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