Resolving Siemens 840D Alarm 700125 on a Hydraulic-Clamped Rotary Axis

Alarm 700125 "Spindle forward feed stop, 10th axis: Pulse or controller release 0 signal" on a SINUMERIK 840D-controlled milling machine is a user-defined alarm generated by the machine builder's PLC logic, not a native NCK alarm. The clearing behaviour described by the field report — a single Alarm Cancel + Cycle Start clears the fault, the axis then resumes motion — is a strong indicator that the NCK has not actually lost the drive enable; rather, the PLC has momentarily revoked a clamping-related interlock and then re-asserted it. The root cause is therefore found in the machine's hydraulic clamping circuit, its pressure-monitoring switch, and the PLC program that ties them to the axis drive enable. This reference walks through the engineering analysis, the PLC interface, the diagnostic flow on the HMI, and the verification steps that close the issue.

1. Alarm 700125 Definition and Scope

On the SINUMERIK 840D platform, alarms in the 700000 series are PLC user alarms. The numeric value 125 within the user-alarm block identifies the specific message; on the affected machine this message reads "Spindle feed stop, 10th axis: Pulse or controller release 0 signal". The message has three key engineering implications:

1. The alarm originates in the PLC, not the NCK. The NCK displays it because the PLC has set a corresponding bit in the NCK-PLC interface.
2. The controller release (German: Reglerfreigabe, signal DB31,..DBX2.1) and/or the pulse enable (signal DB31,..DBX21.7) for the addressed axis have been revoked.
3. The "10th axis" qualifier in the message text tells you the alarm targets a specific axis slot (the 10th configured axis), not the spindle. The slot number is configured in machine data MD20070 and following.

On a retrofit or machine-builder-specific configuration, the slot index is the address in the PLC where the OEM writes the alarm. The user report indicates this is bit DB2.DBX191.1 on the SIMATIC S7 PLC that pairs with the 840D NCU. This bit, when set, causes the HMI to display alarm 700125 with the user-defined message text.

The official description of the NCK-PLC interface for axes/spindles is in the SINUMERIK 840D sl List Manual, section "Signals to/from axis/spindle". Always verify the exact DB numbers for the slot on your specific NCU version (NCU 561.x, 571.x, 710.x or newer).

2. System Topology: 10th Axis with Dual Encoders

The reported machine uses a rotary axis configured as the 10th axis, equipped with two encoders:

• Motor-side incremental encoder — connected to the SINAMICS drive module (e.g. 611U or S120) via the motor encoder port (typically X411 on a 611U or SMI/SMC on an S120). This encoder closes the speed/current loop inside the drive.
• Load-side absolute encoder — mounted on the axis shaft (e.g. Heidenhain RCN, ROD, or EnDat). Wired to the NCK or the drive's second encoder port, this encoder closes the position loop and is used for absolute position recovery after power-off.

Dual-encoder axes on 840D are configured in machine data:

• MD30200 $MA_NUM_ENCS = 2 — number of encoders
• MD30220 $MA_ENC_MODULE_NR — encoder module assignment
• MD32110 $MA_ENC_FEEDBACK_POL — sign of the position feedback
• MD34200 $MA_ENC_REFP_MODE = 0 — reference point approach for incremental

The fact that the alarm targets this specific axis and the description in the field report both point to the axis drive enable. When a hydraulic clamp is closed, the OEM PLC will normally inhibit axis motion to prevent mechanical damage. The alarm 700125 trigger therefore correlates with the clamp/pressure monitoring logic for this axis.

3. Hydraulic Clamping System Architecture

The axis uses an automatic hydraulic clamp that is released before motion and re-engaged after motion. The architecture typically looks like this:

The PLC energises a solenoid valve that routes hydraulic pressure to the clamp cylinder to release the axis. While the solenoid is energised, the PLC expects to see a confirmation: either a digital pressure switch (binary 24 V) that closes when pressure is at or above the clamp-release threshold, or an analog pressure sensor that returns a scaled 4–20 mA / 0–10 V signal. Without this confirmation within a watchdog window, the PLC refuses to issue the controller release for the axis drive, and the NCK raises alarm 700125.

4. Root Cause Analysis Path

The field report documents the following sequence that reliably triggers 700125:

1. Operator commands a motion on the 10th axis (rotary).
2. PLC energises clamp-release solenoid.
3. Hydraulic pump is running (or runs intermittently on pressure demand).
4. Pressure switch briefly drops below threshold.
5. PLC sees "clamp not released" and clears DB31..DBX2.1 (controller enable).
6. NCK raises alarm 700125 because the pulse/controller release chain is broken during motion.

The two engineering paths that can cause a transient pressure dip are:

• Hydraulic leak on the clamp supply line, between the manifold and the return line — confirmed in the field case. As the line bleeds down, the pump cycles on to restore pressure, but the response time of the mechanical system plus the pump's pressure-buildup rate creates a window in which the switch is open.
• Hydro accumulator failure — if the system relies on a bladder- or piston-type accumulator to maintain pressure between pump cycles, a pre-charge loss or rupture means the pump must run continuously and any momentary unloading of the clamp side drops the line pressure below the switch threshold.

The corrective action in the field was a hydraulic line repair between the manifold and the clamp return. After repair, the pressure switch held steady above threshold and the alarm stopped appearing.

5. PLC Interface: DB2.DBX191.1 and STEP 7

The user alarm trigger is referenced in the discussion as DB2.DBX191.1. In SIMATIC S7-300/400 syntax, this is data block 2, byte 191, bit 1. To trace the root cause from the PLC side, you need either:

• A licensed copy of STEP 7 V5.x (S7-300/400) or TIA Portal for S7-1500-based control panels, with the machine builder's project file.
• Access to the HMI's PLC status screen (available on most SINUMERIK 840D HMIs without STEP 7).

On the SIMATIC S7 side, the typical logic looks like:

// Pseudo-ST example: clamp release and controller enable interlock
A "M_CLAMP_REQ_RELEASE"      // request to release clamp
AN "I_PRESSURE_SW_OK"        // pressure switch contact (NO, closes at pressure)
S "F_CLAMP_RELEASED"          // internal flag, clamp released confirmation

// Drive enable chain
A "F_CLAMP_RELEASED"
A "I_SAFETY_DOORS_CLOSED"
A "I_DRIVE_READY"
= "DB31_10.DBX2.1"            // 10th axis controller enable

// User alarm trigger
A "F_CLAMP_RELEASED"
AN "DB31_10.DBX21.7"          // pulse enable feedback from drive
S "DB2.DBX191.1"              // 700125 trigger

The moment the pressure switch contact opens (line pressure falls below threshold), the internal flag F_CLAMP_RELEASED resets, the controller enable drops, the pulse enable feedback turns off, and DB2.DBX191.1 is set. The NCK reacts by displaying alarm 700125 and decelerating the axis on the configured stop ramp.

Open DB2.DBX191.1 in STEP 7 and use Cross References to find every rung that reads or writes it. The rung that sets the bit tells you which condition is failing. The most common culprits, in order of field frequency, are:

1. Pressure switch contact: I_PRESSURE_SW_OK
2. Analog pressure comparator output: F_PRESSURE_OK
3. Solenoid valve command echo feedback: I_SOLENOID_ACK
4. Clamp position limit switch: I_CLAMP_OPEN_LS / I_CLAMP_CLOSED_LS
5. Watchdog timer expired: T_CLAMP_RELEASE

6. HMI-Based Diagnostics Without STEP 7

You can usually inspect the same signals from the operator HMI without a STEP 7 license. On a SINUMERIK 840D HMI (HT8, OP12, PCU50.5):

1. Navigate to Diagnostics → PLC → PLC Status.
2. Enter the data block and byte: DB2, byte 191. The HMI shows the bit pattern in binary.
3. Watch the bits while commanding an axis motion. If bit 1 flickers, that confirms a transient PLC condition; if it latches solid, the condition is sustained.

On a SINUMERIK Operate interface (HMI version 4.5+), the path is Menu Select → Diagnostics → PLC → Variable View. From there you can also force a non-safety input bit (with the correct access level) to verify the alarm responds.

7. Pressure Sensor and Clamp Monitoring

The two pressure-monitoring devices on this class of machine are:

If the analog transmitter is available, route it to a trend on the HMI to confirm the leak hypothesis. A typical signature for a leak between the manifold and the clamp return line is a sawtooth pattern: pressure holds for several seconds, then drops rapidly to near zero over a few hundred milliseconds, then the pump cycles on and restores pressure over 1–3 seconds. The mechanical switch will chatter in this condition, the analog value will show the actual profile, and the PLC's debounce logic may either filter the chatter (good) or pass it through to the alarm (bad — the common case).

Recommended reading on Siemens' documentation set:

• Siemens Industry Online Support — search "SINUMERIK 840D PLC user alarm".
• SINUMERIK 840D sl List Manual (LFB0) — NCK-PLC interface signal reference.
• SINUMERIK 840D sl Programming Manual — PLC programming sections for axis enables.
• SIMATIC S7-300 CPU 31x Manual — data block structure for STEP 7 V5.x.

8. Step-by-Step Diagnostic Procedure

1. Confirm the alarm is PLC-generated, not NCK. Use the HMI alarm log. NCK alarms have a different visual cue (typically a red bar) and a different alarm-clear behaviour. If 700125 clears with one cancel/cycle start, the drive is mechanically fine — the PLC just revoked the enable momentarily.
2. Watch the HMI PLC status for DB2.DBX191.1 in real time. Have an operator command the 10th axis to move while you watch. Record the exact moment the bit sets.
3. Correlate with the hydraulic system. Add a temporary pressure tap or use the analog pressure transmitter (if present) and trend it on the HMI alongside the bit. A sawtooth pressure profile that lines up with the alarm trigger confirms a leak.
4. Check the pump and accumulator health. With the machine in service mode, command clamp release. Watch pump motor contactor and accumulator pre-charge gauge. If the pump runs continuously, the accumulator is suspect. Pre-charge on a nitrogen bladder accumulator is typically set to 60% of the working pressure — for 120 bar working, that is 72 bar N₂.
5. Inspect the clamp supply line. Look for any flexible hose, fitting, or manifold joint between the directional valve and the clamp cylinder. Soap-bubble test or use a UV leak-detection dye under running pump cycles. The field case showed the leak was between the manifold and the return line — the return side bleeds off, the supply side cannot hold, and the switch opens.
6. Verify the pressure switch. Disconnect the switch and test with a hand pump and a reference gauge. Mechanical switches have a hysteresis; confirm the switch's falling threshold is below the pump setpoint (e.g. for 120 bar setpoint and 110 bar nominal, the switch should fall between 95–105 bar to provide a stable margin).
7. Repair and re-test. Replace the leaking hose or seal, refill the accumulator if needed, re-pressurise, and re-run the motion sequence that previously triggered the alarm.

9. Verification and Return-to-Service

After the mechanical repair, run a structured verification:

1. Static test: Energise clamp release, monitor pressure for 60 s. Pressure must stay above the switch falling threshold for the full 60 s.
2. Cyclic test: Command 100 cycles of axis motion with clamp release/re-engage. Monitor DB2.DBX191.1 and the alarm log. Zero 700125 occurrences is the pass criterion.
3. Worst-case test: Run the axis at rapid traverse while a neighbouring heavy-load device (e.g. tool changer) is hydraulically active. The pump should maintain line pressure throughout.
4. Trend verification: Capture a 10-minute pressure trend; the trace should be flat (within ±2 bar) while the axis is idle, and show only expected transients during clamp cycling.

10. Preventive Maintenance Recommendations

• Annual: Check nitrogen pre-charge on the hydro accumulator. Bleed down to the manufacturer's spec and re-charge with dry nitrogen.
• Quarterly: Inspect clamp supply and return lines for chafe, kinks, and fitting tightness. Replace any flexible hose older than 5 years regardless of visible condition.
• Monthly: Verify pressure switch function with a reference gauge; the threshold should not drift more than ±5% from the documented setpoint.
• Weekly: Log the trend of analog pressure values (if equipped) and compare against the previous week's baseline. A drift down indicates a developing leak.
• PLC side: Document the alarm trigger rung (DB2.DBX191.1) in the project comments. Future troubleshooting time is reduced from hours to minutes when the trigger is labelled.

11. Related Alarms and Cross-Reference

Many machine builders group all clamp-related alarms in a contiguous block in DB2. If 700125 points to DB2.DBX191.1, neighbouring bits in the same byte are likely related to other axes' clamps. Document all of them.

12. Field-Engineering Notes and Edge Cases

• Thermal drift: Hydraulic fluid viscosity drops with temperature. On a cold start, the pump may take longer to reach setpoint, and the pressure switch can dip below threshold for several seconds. A warm-up routine that holds the axis clamp released until pressure is stable eliminates this class of false trip.
• Pump unloading valve: Some machines use a load-sensing pump with an unloading valve. If the unloading valve is set too low, the pump may unload during the clamp-release phase, dropping line pressure. Verify the unloading threshold against the machine's hydraulic schematic.
• Schematic accuracy: The field case was prolonged by a poorly translated electrical schematic that omitted a pressure sensor. Cross-check the schematic against the actual wiring on the manifold, and trace every sensor wire back to its PLC input terminal — a one-time exercise that often reveals undocumented devices.
• PLC scan time vs. mechanical response: A PLC cycle of 10 ms is fast enough to catch a 200 ms pressure dip. If you must filter the switch in software, use a 200–500 ms debounce, but verify the filter does not mask a real leak. The best approach is to fix the leak, not to filter the symptom.
• Drive-side checks: If the alarm is reported but the HMI shows the pressure OK and the PLC enable bit is true, suspect the drive-side enable chain: terminal enable on the SINAMICS module, drive safety relay, or PROFIBUS/PROFINET telegram status word bit 1 (control word 1: OFF1). A scope on DB31..DBX2.1 at the PLC output and on the drive's enable terminal will isolate the failing segment.

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