Troubleshooting Sinumerik 840D Simodrive 611D Gantry Trip Alarms 21612, 25040, 25050

Problem Description

This reference covers intermittent gantry-axis trip faults on a Sinumerik 840D-controlled four-axis gantry machine, specifically a Burkhardt Hensel 595/4 stone milling machine driven by Simodrive 611D modular converters. The fault presents as a spontaneous drive-enable cancellation that occurs at any time and any machine position: after minutes or hours of operation, during AUTO mode motion, and even in JOG mode with all axes stationary. The signature alarm sequence typically contains a mix of NC alarms (21612, 25040, 25050) and PLC user alarms (700006 "SM 006 GANTRY AXIS NOT EQUAL" and 700011 "SM 011 REGULATOR FAULT SERVO REGULATOR").

In approximately 80% of events only the PLC user alarms are raised. In the remaining 20% the NC contour-monitoring and standstill-monitoring alarms accompany the PLC stop. The visible cause - a mechanical gantry mismatch - frequently leads operators to inspect the wrong subsystem. The actual root cause is almost always upstream of the gantry comparator logic: a single lead axis (Z on the 595/4) that faults first and propagates a reset signal to the gantry group.

System Architecture

Machine Configuration

Simodrive 611D Topology

The 611D platform combines a shared 6SN1111 bus module with one or more 6SN1123 power modules, each carrying a 6SN1118 control board. Drive enable is the logical AND of three signals:

1. Pulse enable from terminals 663 / AS1 / AS2 (24 V hardware chain)
2. NCK axis enable via PROFIBUS-DP / PROFIdrive control word (bit 0 of STW1)
3. Drive internal readiness (no IGBT fault, no encoder fault, DC-link OK)

When the NCK pulls enable, the 611D executes a parameterized stop (typically 200-500 ms ramp-down) and signals "ready removal" back to the PLC. This is the moment the gantry comparator in STEP 7 typically triggers SM 006.

Sinumerik 840D Gantry Grouping

A gantry grouping is configured in the following machine data (active on 840D / 840D sl):

When the absolute difference between master and slave actual positions exceeds MD37130, NC alarm 21550 "Gantry following axis position difference too large" can be raised. On the Hensel Burkhardt 595/4 this NC alarm is replaced or augmented by the user alarm SM 006 from STEP 7.

Alarm Code Reference

The fault fingerprint on this class of machine has three NC alarm groups and two PLC user alarms.

NC Alarm 21612 - Drive Sign-of-Life Failure

Alarm 21612 in Sinumerik 840D is "Axis %1 drive %2 sign-of-life failure". It fires when the cyclic PROFIBUS-DP / PROFIdrive telegram from the 611D is missing for longer than the configured watchdog window. The NCK then concludes the drive can no longer execute commanded motion and removes axis enable, which is the seed event for the entire fault chain on the 595/4.

Typical root causes:

• Loose or oxidized PROFIBUS connector at the 611D (X421 9-pin D-sub on 6SN1118 board)
• Shield break on the PROFIBUS-DP cable
• Marginal 6SN1118 control board (solder-joint fatigue under thermal cycling)
• EMC interference from a VFD or switching power supply routed too close to the bus
• DC-link sag on the 6SN1111 bus module during regen or peak load

NC Alarm 25040 - Standstill Monitoring

Alarm 25040 is "Axis %1 standstill monitoring". It fires when the actual position leaves the standstill window MD36040 $MA_STANDSTILL_POS_TOL while the NCK expects the axis to be stationary. Typical window size is 0.5-2 mm depending on the axis. On the 595/4, repeated 25040 events on the Z axis during JOG standstill are a strong indicator that the Z axis is the lead-fault source - the Z axis is moving (or being moved by external force) when the NCK believes it is at rest.

NC Alarm 25050 - Contour Monitoring

Alarm 25050 is "Axis %1 contour monitoring" and indicates a following error in excess of the contour tolerance. Relevant machine data:

On a stone mill, mechanical stiffness under heavy cutting load can transiently push the Z axis beyond the contour tolerance. If this happens consistently, the structural or drive gain chain is suspect, not the NCK software.

PLC Alarm 700006 - SM 006 GANTRY AXIS NOT EQUAL

The 7xxxxx range is reserved for PLC user alarms in Sinumerik 840D. Format and trigger condition are defined in the STEP 7 program of the integrated S7-300 PLC. On Hensel Burkhardt machines, "SM 006" is the standard identifier for the gantry comparator path. The signal chain inside the FB is typically:

1. Read actual position difference (Y minus YS) from the NCK axis interface or from the $AA_IM[x] system variable.
2. Compare to a tolerance threshold (often tighter than MD37130 - typically 0.3-0.5 mm on stone mills).
3. Set the SM 006 alarm bit; pull drive enable for the gantry group.

PLC Alarm 700011 - SM 011 REGULATOR FAULT SERVO REGULATOR

SM 011 indicates a servo-regulator fault detected at the PLC level. It is the OR-result of:

• 611D terminal 64 (fault output) assertion
• IGBT fault bit on the 6SN1118 control board
• Motor thermal overload (1FN / 1PH winding PTC evaluated on the 6SN1118)
• Encoder signal lost (1Vpp sin/cos on Heidenhain, or EnDat)
• DC-link undervoltage below 480 V DC on a 400 V mains system

Root Cause Analysis

On the 595/4 gantry stone mill, the typical fault tree in order of probability is:

When only the PLC alarms 700006 / 700011 are raised, the lead axis (Z) is faulting and dragging Y/YS down through the gantry stop chain. The PLC comparator only sees the position divergence after the Z-axis reset has propagated; the Z reset is the upstream event.

Prerequisites for Diagnosis

Before opening the cabinet or going online, assemble:

• Valid STEP 7 / SIMATIC Manager project (LAD or STL) of the machine's PLC program, including the FC/FB that drives the SM 006 and SM 011 alarm paths
• Sinumerik 840D Operator's Manual (lists alarms 21612, 25040, 25050)
• SINUMERIK 840D sl Special Functions (ID 53895023) for the gantry chapter
• SIMODRIVE 611D commissioning manual (6SN1197-0AA60 series)
• 840D alarm log export (CSV or printout) covering the last 20 trips
• HMI access to the Service menu and to the Service display of axis actual values
• PPE and lockout equipment per local electrical-safety procedure

Step-by-Step Diagnostic Procedure

Step 1 - Export the Alarm History

On the HMI navigate to:

Diagnosis > Alarm Log > [Save to file or print]

Capture at least 20 events. For each event record:

• The first alarm raised (root cause candidate)
• The axis that initiated the trip
• The timestamp delta between events (cascading events should be 50-200 ms apart)

Step 2 - Identify the Lead Axis

On the 595/4 the Z axis is the lead-fault axis in the majority of cases. Confirm by sorting events by timestamp and identifying the first non-cascade alarm. Because Sinumerik clears enable on the entire gantry group at the first NC reset, secondary alarms are typically cascade effects, not independent faults.

Step 3 - Pull the PLC Project in STEP 7

Using SIMATIC Manager with the online connection to the integrated S7-300:

1. Open the project matching the machine serial (Hensel Burkhardt ships the source archive with the machine).
2. Go online and force a refresh (PLC > Connect to Target System > Accessible Nodes).
3. Open the alarm-status DB (typically DB2 or a user DB) and inspect the bits corresponding to SM 006 and SM 011.
4. Use cross-reference (right-click > Cross-References) to find the FC/FB that sets these bits.
5. Read the input conditions: actual position difference, drive-ready signals, IGBT fault flags, terminal 64 status.

The relevant inputs are wired to the axis interface DB3x:

Step 4 - Check 611D Drive Status

Read the 7-segment / status display on each 611D module:

Common 611D fault codes for the 595/4 scenario:

Step 5 - Inspect PROFIBUS Cabling

PROFIBUS-DP between the NCK and the 611D modules uses a 9-pin D-sub at the X421 connector on each 6SN1118 control board. Inspect:

• Connector seating (push-pull on metalized hood preferred)
• Shield clamp contact on the D-sub hood (360-degree clamp, not pigtail)
• Cable run for parallel 24 V / 400 V routing (minimum 200 mm separation)
• Termination resistors ON at both ends of the bus segment only
• Pin condition (oxidized or recessed pins cause intermittent dropouts)

Step 6 - Mechanical Inspection of the Z Axis

Even though gantry errors are visible, always inspect the Z axis:

• Ball-screw preload and axial play
• Coupling (jaw, bellows, or disc) for backlash
• Linear guide rail preload and wiper condition
• Motor mounting torque (typical 30-50 Nm on 1PH / 1FN motors)
• Encoder coupling - the most common failure mode on 611D machines
• Cable chain / drag chain stress points (especially at the moving end)

Step 7 - Capture the Drive Signals with SimoCom U

Use SimoCom U (or DriveMonitor for older 611D builds) to read the 611D trace buffer:

• Torque limit reached bit
• Speed controller output
• Following error inside the drive
• IGBT temperature
• DC-link voltage

A repeating torque-limit or following-error spike at the moment of trip confirms the drive is the source. A single undervoltage event at the same timestamp points to the 6SN1111 bus module.

PLC Alarm Decoding in STEP 7

The Hensel Burkhardt SM 006 / SM 011 path is documented inside the FB. To decode it:

1. In SIMATIC Manager go to PLC > Alarm Display to see active user alarms.
2. Right-click SM 006 and choose Cross-Reference.
3. The cross-reference list shows the FBx.DBx byte and bit. The associated network (NWxx) is the comparator.
4. Open the FB in LAD or STL and read the comparator logic.

A typical SM 006 path on Hensel Burkhardt machines is:

// Pseudo-ST for the gantry comparator
gantry_diff := ABS(actpos_y - actpos_ys);
IF gantry_diff > gantry_tolerance THEN
    sm006_alarm := TRUE;
    axis_reset_y := FALSE;
    axis_reset_ys := FALSE;
END_IF;

A typical SM 011 path is:

// Pseudo-ST for the regulator fault
IF NOT drive_ready_x
   OR NOT drive_ready_y
   OR NOT drive_ready_z THEN
    sm011_alarm := TRUE;
    axis_reset_all := FALSE;
END_IF;

When the drive-ready bit drops first and the gantry position difference appears afterwards, the root cause is the drive system, not the gantry mechanics. This sequencing is the key diagnostic discriminator.

Mechanical and Drive System Checks

Linear Encoder and Motor Encoder

The 611D control board supports a motor-mounted encoder (typically ERN1387 / Heidenhain ROD 486 or an EnDat encoder on the 1FK / 1FT motor series). If the encoder ring, glass disc, or coupling is marginal:

1. The drive loses position feedback.
2. The 611D reports encoder error (code 608) on the 7-segment display.
3. The NCK sees the actual position diverge from the setpoint.
4. The PLC comparator triggers SM 006.
5. Drive ready is pulled, SM 011 fires.

Check the encoder cable continuity and connector pin assignment (15-pin on motor side, 25-pin D-sub on the 6SN1118 control board). Replacement cables must be the OEM-specified 6FX8002 series; pin-to-pin compatible cable often violates the shielded-twisted-pair topology required for 1Vpp signals.

Power Section Health

Measure on the 6SN1111 bus module with a calibrated multimeter (powered, no motion):

A weak 6SN1111 bus module - usually identified by bulging electrolytic capacitors or a fading DC-link under load - causes intermittent DC-link dips that look exactly like drive sign-of-life errors to the NCK.

Drive Communication Verification

The 840D communicates to the 611D over PROFIBUS-DP at the standard 12 Mbaud. Configured telegram types are typically Siemens standard telegram 3 (setpoint, actual, 6-bit control/status) or Siemens standard telegram 6 (with torque feedforward). Verify in:

• $MN_DRIVE_TELEGRAM_TYPE[axis] on the NCK
• The same telegram number configured in SimoCom U for the 611D control board
• PROFIBUS slave diagnostics in STEP 7 (no slave-diagnostic alarms present)

If telegram types are mismatched after a service action, the NCK will intermittently throw 21612 because the cyclic data does not match the expected layout.

Bus cycle time is typically 4-8 ms on 840D. The watchdog should be set to at least 3 x bus cycle to ride out normal jitter, but tight to catch a real cable break (typical: 20 ms).

Corrective Actions

After diagnosis, apply the corrective action appropriate to the root cause:

1. Z-axis drive fault - Replace the 6SN1123 power module or the 6SN1118 control board. Clear the NCK alarm. Power-cycle the drive bus. Re-load the drive parameters from the SimoCom U backup.
2. Encoder cable marginal - Replace the motor encoder cable. Use the OEM-specified 6FX8002 cable. Reroute away from power cables. Re-tension the connector backshell to 0.5 Nm.
3. PROFIBUS connector intermittent - Replace the D-sub connector with a Siemens 6GK1500 series PROFIBUS connector. Re-terminate. Verify shield clamp 360-degree contact.
4. Bus module undervoltage - Replace the 6SN1111 bus module. Verify DC-link with the bus disconnected (capacitors should hold 600 V DC for > 60 s after mains removal).
5. PLC comparator threshold too tight - Loosen the threshold in the STEP 7 FB; the recommended value for stone mills is 0.5-1.0 mm. Re-validate gantry alignment.
6. Mechanical play - Reset gantry alignment per the machine commissioning procedure. Tighten couplings to OEM torque. Replace worn ball-screws or guides.

Verification and Commissioning

After the corrective action, perform the following sequence:

1. Power up the machine in service mode. Bring all axes into the safe reference position.
2. Issue NCK reset and acknowledge all alarms.
3. Reference all axes. The reference procedure for Y/YS must be a synchronized reference per $MA_REFP_NC_START_LOCK - the slave cannot start referencing before the master has referenced.
4. Run a controlled circularity test (a 1000 x 1000 mm square in X-Y at feed 2000 mm/min). Monitor the gantry position difference in the Service display: Diagnosis > Service displays > axis actual values. Compute $AA_IM[Y] - $AA_IM[YS]; it should remain below 50% of MD37130 at all points.
5. Run the machine for 8 hours in AUTO mode with a representative stone-milling program. Log the alarm history continuously.
6. If no alarms recur, the corrective action was successful.

Acceptance Criteria

Prevention and Monitoring

• Add a daily gantry position-difference check to the operator log. The Service display readout takes 30 seconds and identifies drift before it triggers SM 006.
• Use SimoCom U trace recording during commissioning to capture 611D internal signals. Archive the trace file with the machine documentation.
• Replace PROFIBUS connectors every 5 years as preventive maintenance. The contact springs lose tension with thermal cycling.
• Keep 611D firmware on a known-good version across the entire gantry group. Mixed SW versions cause 21612 under heavy bus load.
• Maintain a running count of the 700006 and 700011 alarms (can be done with a small STEP 7 counter). A rising trend warns of a developing fault.
• Inspect the encoder cable drag chain every 12 months; replace at first sign of jacket damage.

Edge Cases and Field-Commissioned Caveats

• If 21612 appears only on the Z axis, the 6SN1118 control board on the Z drive is the most likely culprit. Swap with the X drive as a sanity test - if the fault follows the board, replace it; if it stays with the Z axis, the motor or encoder is suspect.
• If the 6SN1111 bus module shows > 8% DC-link ripple under no load, replace the module even if it has not yet thrown a fault code. The electrolytics are end-of-life.
• If a 25050 contour-monitoring alarm on Z correlates with a heavy cut start (a 0.5 s impulse), the KV factor (MD36030) is too low. Increase in 0.05 increments and re-validate.
• If the gantry position difference grows with ram travel (Y axis extended), the Y-axis linear guide is wearing; mechanical inspection takes priority over PLC tuning.