Resolving SINUMERIK 840D Alarm 20122 Compensation Table 5 Error

Resolving SINUMERIK 840D Alarm 20122: Compensation Table 5 Invalid Axis Assignment

Alarm 20122 "Compensation table 5: invalid axis assignment" is a configuration-level alarm that fires on Siemens SINUMERIK 840D / 840D sl controllers whenever the NCK boots, performs a warm restart, or activates a channel whose axis list does not match the axis index stored inside a compensation table. The alarm clears with NC Reset, which often makes it look "harmless" during early commissioning, but it is a hard configuration error that prevents unattended start-up and must be resolved before the machine can be released for production. This reference covers the underlying compensation-table architecture, the exact parameter set that controls it, the three valid resolution paths, and the multi-channel behavior that triggers the alarm when a second channel is enabled with axes that table 5 cannot reference.

1. Alarm 20122 Problem Summary

Alarm 20122 belongs to the SINUMERIK alarm range 20000-20999, which is reserved for compile-time and run-time errors in NCK configuration data. The alarm is delivered with channel context and a table index, and the standard alarm text from the SINUMERIK alarms documentation is:

Channel %1: Compensation table %2: invalid axis assignment. The compensation table has been assigned an axis that does not exist or is not configured to use compensation.

The behavior described in the field — "alarm appears on every restart but disappears after Reset" — is exactly the expected behavior of a configuration alarm. NCK performs an internal consistency check on compensation tables at channel activation. If the table references a non-existent or compensation-disabled axis, the NCK raises 20122. NC Reset clears the display, but the next channel activation re-runs the check, so the alarm re-appears. The Reset-only workaround is not acceptable for production release; the configuration itself must be corrected.

2. SINUMERIK 840D Compensation Table Architecture

SINUMERIK 840D supports several types of geometric and thermal compensation that all share the same data structure: an axis-indexed compensation table stored as a series of (position, compensation value) points. The NCK can hold up to eight (or sixteen with the extended option) such tables per channel.

All cross-compensation (CEC) tables share a single set of enabling setting data: $SN_CEC_TABLE_ENABLE[n] and $SN_CEC_TABLE_AXIS[n], where n is the table index 0..7 (0..15 with extended option). Each table is permanently assigned to one specific machine axis in its channel through these two setting data entries. The compensation values themselves live in $NN_CEC_TABLE[n,0..7] and the per-table scaling / offset in $SN_CEC_TABLE_OFFSET[n] and $SN_CEC_TABLE_FACTOR[n].

3. Root Cause Analysis of Invalid Axis Assignment

Alarm 20122 fires for one of four reasons, all of which are essentially the same root condition — the table's axis index does not resolve to a configured, compensation-enabled axis in the active channel:

1. Axis index out of range: $SN_CEC_TABLE_AXIS[5] contains a number that is not a valid machine axis in the current channel (e.g., the table still references axis 7 from an old config, but the channel has only 5 axes).
2. Axis not assigned to the channel: The axis index is valid on the system, but the axis is assigned to channel 1 while the compensation table is being evaluated in channel 2 (or vice-versa).
3. Axis compensation globally disabled: $MA_CEC_ENABLE[AX] for the referenced axis is set to 0, so even though the table "points" at a valid axis, the NCK refuses to bind the table to a compensation-disabled axis.
4. Cross-channel pollution: The compensation table data was copied from another channel's image where the axis indices were 1..6 (channel 1) and the table was loaded into channel 2 without re-mapping the axis index.

The most common field scenario — "alarm appears when I enable channel 2" — is almost always scenario 4: a commissioning engineer copies a multi-axis configuration that includes an active compensation table (e.g., for vertical-axis sag or leadscrew error) but the table still references an axis from channel 1's geometry. When channel 2 is activated, the NCK re-validates the table and rejects the cross-channel axis index.

4. Key Parameters and Data Words

The complete parameter set that must be inspected and corrected is summarized below. Pay attention to the prefix — it determines whether the parameter is NC-wide, axis-specific, or channel-specific.

Note that setting data items use the $SN_ prefix and live at the channel level, so they are per-channel. If the same compensation must apply to channel 1 and channel 2, each channel must have its own correctly-assigned $SN_CEC_TABLE_AXIS[n] entry. The table values stored under $NN_CEC_TABLE are NC-wide.

5. Locating $SN_CEC_TABLE_ENABLE in the HMI / NC

The most common reason field engineers "cannot find" $SN_CEC_TABLE_ENABLE is that the HMI display filter is set to a category that excludes setting data, or the access level (password) is insufficient. Use the following sequence to surface the parameter set on a stock SINUMERIK Operate install:

1. On the HMI, navigate to Setup > Compensation. For older HMI Advanced, use Commissioning > NC > Setting Data.
2. If the screen is empty, press the softkey Display MD and change the filter to Setting Data ($SN_).
3. Open the filter dialog and enter CEC_TABLE in the search box — this filters all $SN_CEC_TABLE_* entries.
4. If nothing appears, the active access level is too low. Press the Access level softkey and switch from Operator to Service or Manufacturer (password required; default manufacturer password on commissioning is SUNRISE, service CUSTOMER — change immediately on production hardware).
5. Alternatively, write the parameter directly from an NC program: SETAL("CEC_TABLE_ENABLE[5]",0), or via PLC FB7 ("PI Service") to set the setting data remotely.

Once the parameter is visible, record the current value of $SN_CEC_TABLE_ENABLE[5], $SN_CEC_TABLE_AXIS[5], and the corresponding $MA_CEC_ENABLE on the axis referenced by the table. These three values together diagnose whether the table is configured correctly.

6. Solution Path A — Correct the Axis Assignment in Table 5

If the compensation is genuinely required (most commonly: vertical-axis sag, leadscrew compensation on a long-travel axis, or cross-coupling between gantry axes), the correct fix is to re-bind table 5 to a valid, channel-resident, compensation-enabled axis.

1. Decide which axis in the channel needs the compensation. For a vertical-axis sling, this is typically the Z axis. For leadscrew error, it is the longest-travel linear axis. For gantry cross-coupling, it is the slave axis of the gantry pair.
2. Verify that $MA_CEC_ENABLE[AXn] = 1 on the chosen axis. If it is 0, change it to 1 and trigger an NCK reset (or click Activate MD on the HMI).
3. Set $SN_CEC_TABLE_AXIS[5] to the axis index. In multi-channel systems, axis indices are NC-wide: axis 1 = first machine axis on the system, axis 2 = second, etc. The axis index is NOT the channel-local axis number; it is the global axis index as listed in $MN_AXCONF_MACHAX_NAME_TAB and visible under Axis configuration.
4. Confirm $SN_CEC_TABLE_ENABLE[5] = 1.
5. If using scatter or sling assignment instead, ensure $MA_CEC_SLING_ASSIGNM[AXn] = 5 or $MA_CEC_SCATTER_ASSIGNM[AXn] = 5 matches the table number.
6. Trigger an NC Reset. The alarm should not re-appear on subsequent resets or channel enables.

7. Solution Path B — Disable Table 5 via $SN_CEC_TABLE_ENABLE

If the compensation was inherited from a previous configuration and is not actually needed in the new channel layout, the table can simply be disabled. This is also the correct remediation if the customer cannot provide the original commissioning sheet for the compensation.

1. Set $SN_CEC_TABLE_ENABLE[5] = 0.
2. Optional but recommended: set $SN_CEC_TABLE_AXIS[5] = 0 to flag the table as unassigned. A non-zero value on a disabled table is harmless but can confuse later engineers reading the data.
3. Trigger an NC Reset. Alarm 20122 should clear immediately and not return on the next boot.
4. To make the change survive a Power-On, perform Commissioning > Save data > NC active data from the HMI, or set $ON_BACKUP_REQUIRED = 1 and run the PI service _N_BACKUP. Skipping this step causes the setting data to revert to the loaded image after a Power-On, and the alarm will reappear on the next cold start.

If the alarm reappears after Power-On despite setting $SN_CEC_TABLE_ENABLE[5] = 0, the issue is almost always that the value was changed but not backed up. Setting data modifications are volatile in NCK memory until the backup is performed.

8. Solution Path C — Reset Behavior and NCK Persistent Storage

The behavior described in the original field report — "alarm appears on every restart but disappears after Reset" — is consistent with a setting-data configuration error that has been loaded into the NCK image (loadable backup) but where the in-RAM image has been modified. To permanently resolve the condition without disabling the table:

1. Identify which of Solution A or Solution B is appropriate (see sections 6 and 7).
2. Apply the change in the active NCK image.
3. Press NC Reset — alarm clears.
4. From the HMI, navigate to Commissioning > Save data > NCK active data, or invoke PI _N_BACKUP from the PLC via FB7.
5. Perform a controlled NCK Power-On to verify the change persists.

Without step 4, the alarm will indeed reappear after the next Power-On, which is the behavior the original field engineer observed.

9. Multi-Channel Configuration Considerations (Channel 2 Specific)

SINUMERIK 840D supports up to ten channels per NCU, and compensation tables are channel-scoped. The behavior "alarm appears only when I enable channel 2" is therefore expected if:

• The compensation table is enabled in channel 2 but its axis index resolves to a channel-1 axis that is not part of channel 2's geometry.
• The original commissioning left $SN_CEC_TABLE_ENABLE[5] = 1 globally while the table was meant for channel 1 only.
• The axis reference index in $SN_CEC_TABLE_AXIS[5] refers to a spindle or a positioning axis that is not assigned to channel 2.

The corrective pattern is to use channel-specific configuration: read the table data in the channel where it is meant to apply, and set $SN_CEC_TABLE_ENABLE[5] = 0 on the channel that does not need it. Because setting data is channel-local, you can have $SN_CEC_TABLE_ENABLE[5] = 1 on channel 1 with a correct axis index, and $SN_CEC_TABLE_ENABLE[5] = 0 on channel 2, and the NCK will not complain on channel 2 activation. Use the HMI to switch channels (Channel switch softkey, or click the channel name in the status bar) and confirm each channel's setting data separately.

10. Verification Procedure and Commissioning Checks

After applying any of the three solutions, perform the following verification sequence to confirm the alarm is permanently cleared:

1. NC Reset — observe alarm log. 20122 should be marked Cancelled.
2. Disable and re-enable the affected channel from the HMI (Channel state menu). No new alarm should appear.
3. Power-On the NCK (cold restart). After NCK comes back up, no 20122 should appear at any of: NCK ready, channel activation, channel reset.
4. Display the active compensation state under Diagnostics > Comp. table state or by reading $AC_CEC_TABLE_STATE in the NC program. State should show "active" if the table is enabled, "inactive" if disabled — never "invalid axis".
5. Run a referencing sequence and a simple linear move on the axis that uses compensation. If the compensation is active, you can verify it by plotting actual vs. nominal position via Trace and looking for the expected compensation curve.
6. Save the commissioning changes via Commissioning > Save data > NC active data. This is required for the fix to persist across Power-On.
7. Archive the resulting image and update the machine documentation: record the table number, the axis it's bound to, the compensation values, and the SD snapshot date.

11. Related Compensation Alarms and Cross-Reference

Alarm 20122 is part of a family of compensation-related alarms in the 20000-20999 range. When investigating, it is worth scanning the alarm log for siblings that often co-occur:

If 20122 is accompanied by 20130, fix $MA_CEC_ENABLE first (it is a precondition) before addressing the table-axis binding. For full reference, see the official SINUMERIK 840D sl Diagnostics Manual and the SINUMERIK 840D sl Parameter Lists Manual which provide the definitive parameter database.

12. Frequently Asked Questions