27006

Check MCP Single Block Key Status

Examine the machine control panel for illuminated SBL key or check HMI Operating Area > Machine > Operating Mode display for active Single Block indicator.

On OP015T/OP019, the SBL key has an integrated LED that illuminates when active.

Verify HMI Single Block Setting

Navigate to HMI > Machine > Operating Modes and check if 'Single Block' checkbox is selected in the execution options.

The SBL mode can be toggled via softkey SK4 'Single Block' in most SINUMERIK HMI versions.

Deactivate Single Block Mode

Press the SBL key again to deactivate or uncheck Single Block option in HMI to clear DB21.DBX1.4 signal and resume continuous execution.

Ensure program position and workpiece setup are correct before resuming automatic mode.

Monitor PLC Interface Signals

Access Diagnostics > PLC > Variable Status and monitor DB21.DBX1.4 (TO_NCK_SBL) to verify if PLC is forcing the signal active.

Use PLC program status to trace which FB/FC blocks are writing to the SBL interface.

Check Safety Interlock Logic

Review PLC safety logic in OB1 for conditions that automatically activate SBL mode during fault recovery or after E-STOP reset sequences.

Do not bypass safety interlocks - identify and resolve the underlying safety condition first.

Analyze Custom Machine Logic

Examine machine manufacturer's PLC code for automatic SBL activation during tool change, workpiece setup, or after alarm conditions in the NC program.

Check for timer-based SBL activation that may be part of the machine's restart sequence.

Check Single Block Machine Data

Navigate to Commissioning > Machine Data > Channel and examine MD20734 $MC_SINGLE_BLOCK_MASK for automatic SBL activation settings.

MD20734 bit settings determine which conditions automatically trigger SBL mode.

Verify Channel Operating Mode Settings

Check Commissioning > Machine Data > Channel > General for any forced operating mode parameters that may default to Single Block execution.

Changing machine data requires SINUMERIK manufacturer access level (password level 3).

Review Restart Behavior Parameters

Examine MD20700 series parameters for program restart behavior that may automatically enable SBL mode after interruptions or alarm conditions.

Some machine builders configure automatic SBL activation after tool breakage or collision detection alarms.

Review Recent Alarm History

Access Diagnostics > Alarms > Alarm Log and check for recent critical alarms (25xxx, 26xxx series) that may trigger automatic SBL activation.

Look for collision detection alarms (25050-25070) or axis following error alarms that often force SBL mode.

Check Emergency Stop Recovery Status

Verify if the system recently recovered from E-STOP condition and examine machine-specific restart procedures that may enforce SBL mode.

Complete all required reference point approaches before attempting to clear SBL mode after E-STOP recovery.

Perform Safe Program Restart

Execute several program blocks in SBL mode to verify correct axis positioning and program flow before deactivating Single Block mode.

Use dry run mode (DRY) in combination with SBL for additional safety during restart verification.

Check HMI-NCK Communication Status

Navigate to Diagnostics > Communication and verify stable connection between HMI and NCK with no timeout errors or data exchange failures.

Look for cyclical communication alarms in the 10xxx or 11xxx series that may affect mode switching.

Restart HMI Application

Perform controlled HMI restart via Service > Restart HMI to clear any corrupted operating mode status variables.

Save any active program data before restarting HMI to prevent data loss.

Verify Operating Mode Synchronization

After HMI restart, check that operating mode display correctly reflects actual NCK status and SBL key functionality responds properly.

Toggle between different operating modes (AUTO, MDI, JOG) to verify proper HMI-NCK synchronization.