What causes Siemens SINUMERIK alarm 28020?

Parameter or machine data configuration error. Hardware fault on the NC or drive component. Part program error or illegal command. Communication fault between NC and drive or I/O modules. Software or firmware error requiring system restart.

How do I fix Siemens SINUMERIK alarm 28020?

1. Note the complete alarm text including any axis, channel, or block number values. 2. Check for related alarms active simultaneously in the SINUMERIK alarm list. 3. Power cycle the machine to clear transient faults. 4. Review the SINUMERIK machine data for parameters related to the alarm. 5. Consult the Siemens SINUMERIK alarm manual for alarm 28020 detailed remedy.

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Siemens SINUMERIK Controller

Communication · Intermediate · 30-120 minutes

Alarm 28020

Siemens 840D 28020 - NCU link: too many link axes configured

Alarm 28020 indicates the NCU link communication interface has exceeded its bandwidth capacity for the configured number of link axes. The NCU link protocol has a finite number of communication slots available, and the current machine axis configuration defined in $MN_AXCONF_LOGIC_MACHAX_TAB and $MN_AXCT_AXCONF_ASSIGN_TAB containers exceeds this limit.

Siemens SINUMERIKCommunicationIntermediate30-120 minutes

28020

Siemens 840D 28020 - NCU link: too many link axes configured

NCU link: too many link axes configured

Root Cause Summary

Alarm 28020 indicates the NCU link communication interface has exceeded its bandwidth capacity for the configured number of link axes. The NCU link protocol has a finite number of communication slots available, and the current machine axis configuration defined in $MN_AXCONF_LOGIC_MACHAX_TAB and $MN_AXCT_AXCONF_ASSIGN_TAB containers exceeds this limit.

Safety

Refer to Siemens documentation
Ensure qualified technician performs repairs

Causes and Fixes

1
Access the NCK Parameter menu via HMI: Diagnostics > Parameters > Machine Data. Navigate to category 'Axis Configuration' and review $MN_AXCONF_LOGIC_MACHAX_TAB to count the total number of configured link axis entries.
2
Verify the container assignments in $MN_AXCT_AXCONF_ASSIGN_TAB1, $MN_AXCT_AXCONF_ASSIGN_TAB2, etc. Cross-reference each axis assignment against the NCU link physical drive count (typically 4-8 drives maximum per NCU link module depending on 840D hardware version).
Ensure you are reviewing the ACTIVE container set, not draft/backup MD sets.
3
If axis count exceeds NCU link capacity, remove non-essential link axes from the configuration. Reduce entries in $MN_AXCONF_LOGIC_MACHAX_TAB by commenting out or deleting rows, then reload the NC configuration via PLC > Restart > Warm Start.
Removing link axes may disable dependent G-code functionality; coordinate with programming team.
4
Confirm the alarm clears after reboot by triggering a Diagnosis > Service > Clear Alarms cycle and initiating a POWER ON sequence.

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