Alarm 25000 Overview and Engineering Significance
Alarm 25000 "Active encoder hardware fault" is one of the highest-priority diagnostics on the SINUMERIK 840D / 840D sl platform. It is raised by the NCK when the active position measuring circuit (measuring system 1 or measuring system 2) cannot deliver a valid, cyclically-updated position value. The controller immediately withdraws axis enable, opens the drive's pulse enable, and brings every referenced axis in the affected channel to a controlled stop.
On a Special Purpose Machine (SPM) with two vertical slides configured in two independent channels, Alarm 25000 appearing simultaneously on both axes almost always points to a shared upstream resource, not to two independent component failures. The shared resource is typically the encoder power rail, the drive bus, the NCU measuring-circuit module, the cable shield/ground reference, or a corrupted machine datum.
The alarm is part of the SINUMERIK ID list (FB1 / FB2 error codes). The full ID number printed in the alarm line is 25000 000 with the measuring-circuit index appended as the suffix (e.g., 25000 001 for measuring system 1 on the first axis). The sub-code drives the operator-action suggestion shown on the HMI and is essential when requesting Siemens support.
840D NCU and 611D Drive Hardware Architecture
Before replacing parts, confirm exactly which generation of hardware is installed. The fine boring machine in the source case is described as 840D PL (Powerline) with SIMODRIVE 611D, which is the pre-2008 generation. The PowerLine topology differs materially from 840D sl / SINAMICS S120.
| Component | PowerLine (PL) Generation | Solution Line (sl) Generation |
|---|---|---|
| NCU module | NCU 561.x, 571.x, 572.x, 573.x | NCU 710.x, 720.x, 730.x |
| Drive | SIMODRIVE 611D (6SN11xx) | SINAMICS S120 (6SL3xxx) |
| Drive bus | Parallel 611D bus ribbon cable | DRIVE-CLiQ (Ethernet-based) |
| Encoder interface | Sub-D / terminal connector on drive module | DRIVE-CLiQ on SMC/SME/SMI |
| Typical encoder | 1 Vpp sin/cos or EnDat on motor | EnDat 2.2 via DRIVE-CLiQ |
| Boot state check | NCU status 6 = OK | NCU status 7 / green LED |
On the 611D PowerLine platform, the motor encoder cable terminates on the control module (control card) of the SIMODRIVE 611D drive, not on the NCU. The NCU only sees the position value after the drive has decoded the sin/cos signals and forwarded them over the 611D bus. This single fact dictates the diagnostic order: encoder → cable → control card → drive bus → NCU.
NCU Status LEDs (PowerLine)
| 7-Segment Display | State |
|---|---|
| 0 | Reset |
| 1 | Boot loader active |
| 2 | BIOS / firmware loading |
| 5 | NCK software loading |
| 6 | NCK ready, awaiting PLC / drive |
| 7 | System in cyclic operation |
| 8..F | Fault - reference Siemens manual |
A status of 6 indicates the NCK is healthy and waiting for the drive bus to come up, which rules out the most basic NCU failure - it does NOT rule out a defective measuring-circuit module on the NCU side or a corrupted machine datum.
Encoder Topology: Active vs Passive Measuring Systems
Every axis in SINUMERIK has two configurable measuring circuits:
- Measuring system 1 (passive): Indirect measurement via the motor-mounted encoder. Provides position feedback through the drive. Used for following error, speed control, and as the active feedback source on most SPM machines.
- Measuring system 2 (active): Direct measurement on the load (linear scale, rotary encoder on the spindle nut, or external glass/steel scale). On a fine boring head, this is typically the boring-head diameter-position feedback or a linear scale on the slide.
The alarm 25000 applies to the currently active measuring system, not necessarily to the motor encoder. If the active system in MD30200 $MA_ENC_IS_LINEAR[AX2] / $MA_NUM_ENCS is set to use a direct measuring system, the alarm will fire even when the motor encoder is perfect.
active measuring system 1 or active measuring system 2 for each axis. The motor encoder was reported as active in the source case, which directs the diagnostic toward the motor/encoder/cable/control-card chain.Measuring System Selection Machine Data
| MD | Name | Typical Value | Effect |
|---|---|---|---|
| MD30200 | $MA_ENC_IS_LINEAR[n] | 0 (rotary) | Defines encoder type |
| MD30230 | $MA_ENC_INPUT_NR[n] | 1 or 2 | Connector assignment |
| MD32100 | $MA_ENC_FREQ_LIMIT | Encoder-dependent | Max input frequency before fault |
| MD36300 | $MA_ENC_FREQ_LOW | Encoder-dependent | Low-frequency limit for sim/cos |
| MD38000 | $MA_MD_FILE_TYPE | - | Archive marker |
Root Cause Analysis for the Source Case
The reporter has already executed the most common shotgun-replacement steps: control card, encoder cable, motor bench trial. Despite this, Alarm 25000 persists on both channels. The probable-cause matrix below is ordered by likelihood given the evidence presented:
| Rank | Probable Cause | Evidence Supporting | Evidence Against |
|---|---|---|---|
| 1 | NCU measuring-circuit module defect (one shared encoder-input sub-board serving both axes) | Both axes fail simultaneously; card swap did not help; spare cards also came from a running machine but may not be fully compatible | NCU status 6 OK |
| 2 | 611D drive bus ribbon / terminator issue | Both axes read no encoder; replacing control cards did not help | Drive is reachable (axes respond to disable) |
| 3 | Encoder cable shield ground reference or shared encoder 5 V return path | Earth-to-neutral 0.6 V is elevated; both axes share the same cable tray | Cables were replaced |
| 4 | MD32020 $MA_JOG_INC_MODE or measuring-circuit selection MD mis-set after a recent archive load | Both axes fail in same way; archive reload was suggested as step #1 | No archive work reported |
| 5 | Active vs passive selection inverted in NC program or NCK MD | Could fire alarm on motor encoder even if motor encoder is fine | Motor bench trial OK |
| 6 | Drive firmware mismatch between replacement control card and NCU expected version | Both axes fail; replacement card came from another machine | Original card also failed |
| 7 | Sub-D connector pin damage at drive side (encoder cable) | Continuity check may pass while pin-socket contact is intermittent under vibration | Cable replaced |
| 8 | Mechanical over-travel that ripped a wire inside the encoder | Both axes simultaneously - improbable on same machine cycle | Motor bench trial OK |
For the source case, ranks 1, 3, and 4 are most consistent with the evidence. Ranks 5 and 6 are diagnostic-only checks and cost nothing to execute.
Pre-Troubleshooting Safety and Access Checklist
- Lock out and tag out (LOTO) the main disconnect. Bring the cabinet to a safe state with the 24 VDC still present for diagnostics.
- Note the NCU software version on the HMI: Diagnostics → Service Display → Version. Typical 840D PL versions are
SW 6.4,SW 7.4, orSW 7.6. Siemens support requires the exact build. - Capture the active alarm list: Alarm → Alarm Log. Screenshot both the alarm number line and the clear-time axis.
- Capture the service axis screen for both channels: Diagnostics → Service Display → Service Axis.
- Confirm you have a complete machine archive (NC + PLC + drive) backed up to USB before any reload attempt.
- Locate the encoder wiring diagram in the cabinet documentation. On a 611D system the encoder cable shield is bonded at one end only (drive side) and the encoder housing is bonded via the connector backshell.
Step-by-Step Diagnostic Procedure
Step 1 - Validate the Active Measuring System
On the HMI navigate to:
- Menu Select → Diagnostics
- Service Display
- Service Axis
- Select each axis in turn.
- Read the field "Active measuring system". The display must show
1or2matching MD30230.
If the active field shows 2 and the machine only has a motor encoder, you have found the root cause - the NC is configured to use a non-existent measuring system. Set MD30200 / MD30230 to the correct motor-encoder channel, or change $MA_ENC_FREQ_LIMIT accordingly. Reload the archive if the MDs appear correct but the runtime disagrees.
Step 2 - Reload the NC/PLC Archive
A corrupted NCK archive can change measuring-circuit MDs silently. Perform a full NC + PLC + drive archive reload from a known-good source:
- Insert the USB archive into the HMI's USB port.
- Menu Select → Commissioning → Series Commissioning → Read In Archive
- Select the matching archive file and confirm.
- Allow the system to perform a full restart (status 6 → 7).
Step 3 - Verify Encoder Cable Continuity and Shield Termination
The reporter checked 5 VDC at the encoder end. That only confirms the supply rail, not signal integrity. Perform the full sweep:
| Pin (motor end Sub-D, 17W2 typical) | Pin (drive end) | Expected Resistance | Pass Criteria |
|---|---|---|---|
| A1 (Cos+) | A1 | < 1 Ω | Continuity, no intermittency when flexed |
| A2 (Cos-) | A2 | < 1 Ω | Continuity |
| A3 (Sin+) | A3 | < 1 Ω | Continuity |
| A4 (Sin-) | A4 | < 1 Ω | Continuity |
| 1 (5V) | 1 | < 1 ⎈ | With load 0.5 A |
| 2 (GND) | 2 | < 1 ⎈ | Same |
| Shell | Shell | < 1 ⎈ | Shield bonded at drive end only |
Then check insulation: every signal conductor against every other signal conductor and against ground must show > 10 MΩ. Use a 500 V megohmmeter - a low-voltage ohmmeter can miss insulation breakdown under drive noise.
Step 4 - Oscilloscope the Encoder Signals at the Drive Card
With the machine in a safe state but with control voltage present, command a slow JOG and observe the cos and sin differential pairs at the drive card connector (back-probe the Sub-D). Spec limits for 1 Vpp sin/cos encoders:
| Parameter | Spec |
|---|---|
| Differential amplitude A | 0.8 Vpp to 1.2 Vpp |
| DC offset (each leg to 2.5 V reference) | 2.3 V to 2.7 V |
| Cos-Sin phase | 90° ± 5° electrical |
| Frequency at JOG | Below MD36300 limit |
Amplitude drop-out, ringing > 20% overshoot, or a frozen DC offset indicates a cable or connector issue even when the cable is "new." New cables can be defective from the factory, especially after the cable has been dragged around during installation.
Step 5 - Drive Bus and Control Card Isolation
Both axes failing after independent control-card swaps points at the drive bus. Inspect the 611D ribbon cable between drive modules and NCU for bent pins, contamination, or cracked insulation. Confirm the bus terminator at each end is present (611D requires terminators at both ends of the bus). Power down before reseating any ribbon.
Inline SVG: 611D Bus Topology
Step 6 - NCU Measuring-Circuit Module Swap
If Steps 1-5 do not isolate the fault, the NCU's measuring-circuit sub-board is the next suspect. The NCU 573 and 572 have plug-in measuring modules (Siemens part numbers 6FC5357-0BAxx depending on variant). Swap with a known-good spare, transfer the firmware card, and verify the boot sequence returns to status 7.
Step 7 - Cabinet Ground and Bonding Audit
An Earth-to-Neutral voltage of 0.6 V is on the high side for a 400 V industrial cabinet. Encoder reference relies on a clean < 50 mV chassis-to-encoder-shield return path. Audit:
- Star-ground point of the cabinet (PE bar) - measure resistance to system earth rod, target < 1 Ω.
- Encoder cable shield bonded only at drive end via 360° backshell clamp.
- Encoder cable separated from motor power cable (min 200 mm parallel, 90° crossings).
- No intermediate junction boxes between drive and motor - splices are common failure points even when "checked."
Drive Module LED and Alarm Diagnostics
The 611D control module exposes a 7-segment and several LEDs. The reporter's screenshot was taken without an active drive alarm LED. Capture the LEDs with the drive in fault state:
| LED | Color / State | Meaning |
|---|---|---|
| H1 (Status) | Green steady | Drive ready |
| H1 (Status) | Green flashing | Drive in fault, see sub-code |
| H1 (Status) | Red | Hardware fault on the control card itself |
| H2 (Bus) | Green steady | Cyclic bus communication |
| H2 (Bus) | Off | Bus disconnected |
A red status LED rules out "the encoder is bad" and confirms the control card or its 24 V supply is the problem. With both axes red, the shared resource is upstream of the drive - measure the 24 VDC supply to each module and confirm the ribbon cable is not shorted.
Related Siemens Alarm Codes
While chasing 25000 you will also see the following correlated alarms. Treat each as a clue, not as a separate fault:
| Alarm | Meaning | Relationship to 25000 |
|---|---|---|
| 25000 | Active encoder hardware fault | Primary |
| 25001 | Passive encoder hardware fault | Same axis, redundant path |
| 25010 | Encoder contamination or zero-mark error | Often co-fires with 25000 |
| 25201 | Drive fault - hardware | Drive-side root cause |
| 300500 / 380500 | Axis enable missing | Resulting from 25000 |
| 8020 | Speed controller output limited | Resulting from 25000 |
| 21612 | Channel reset by PLC | Reset storm - if PLC is cycling because of drive missing |
Verification and Return to Production
- Clear all alarms via Alarm → Clear or via PLC reset bit.
- JOG both axes in slow override (5%) - verify no alarms in either direction.
- JOG with rapid override (100%) for at least one full travel - this exercises the encoder frequency limit MD36300 / MD32100.
- Execute an NC program on each channel independently - some faults only show under closed-loop following error.
- Reference both axes - successful referencing proves the encoder zero mark is readable.
- Run a full part on each slide for at least 30 minutes, observing the alarm log.
- Save a fresh NC + PLC archive including the fix to USB.
Field-Escalation Checklist for Siemens Support
If the fault survives Steps 1-7, gather the following data before contacting Siemens support or shipping the NCU to the repair center:
- NCU 7-segment status at the moment of fault.
- Service Axis screenshot for both axes showing the active measuring system field.
- Full alarm log with timestamps exported as text.
- Drive firmware version of the original and replacement control cards (printed on the bar-code label).
- NCK software version (from Service Display → Version).
- Encoder part number (motor OEM label, e.g. Heidenhain EQN 1325 or ERN 1387).
- Photo of the 611D control module LED state.
Preventive Recommendations After Recovery
- Add encoder cable continuity as a quarterly PM using the table in Step 3.
- Bond the cabinet star-ground to a dedicated earth rod, target < 1 Ω.
- Adopt the rule "no encoder splice boxes" - run a single continuous shielded cable from motor to drive.
- Maintain a current NC + PLC + drive archive on USB in the cabinet, dated.
- Photograph all MDs in MD lists → All machine data immediately after commissioning and store with the archive.
FAQ
What does Siemens 840D NCU Alarm 25000 mean exactly?
Alarm 25000 "Active encoder hardware fault" indicates the NCK is not receiving a valid position value from the encoder configured as the active measuring system for the affected axis. The drive pulse enable is withdrawn and axis motion stops. The sub-code (25000 001 or 25000 002) tells you which measuring circuit is failing.
Can Alarm 25000 appear on two axes at once for a single root cause?
Yes. On 840D PL systems with 611D drives, two axes failing with 25000 simultaneously usually points to a shared resource: the NCU measuring-circuit module, the 611D drive bus, a common encoder supply rail, or a corrupted NCK archive. Replace one-axis components and re-test before swapping the NCU.
How do I confirm which encoder is "active" on a Siemens 840D axis?
Use HMI menu Diagnostics → Service Display → Service Axis. The "active measuring system" field will show 1 or 2. Compare it against MD30200 $MA_ENC_IS_LINEAR and MD30230 $MA_ENC_INPUT_NR. If the runtime disagrees with the MDs, reload the NCK archive.
Why does the motor bench-trial test pass but Alarm 25000 still fires in the machine?
A bench trial only confirms the motor spins and the encoder delivers a signal. It does not exercise the cable routing inside the cabinet, the drive-side connector, the drive control card encoder input stage, or the NCU measuring circuit. The fault can live in any of those segments even with a good motor.
When is it correct to ship the NCU to Siemens for repair because of Alarm 25000?
Escalate when: the active measuring system is verified, the encoder cable passes continuity and insulation, the drive control card is known good, the 611D bus ribbon and terminators are verified, the encoder signals at the drive card meet 1 Vpp spec on a scope, and the alarm still fires on both axes. With both axes failing in the same way and clean everything else, the NCU measuring-circuit sub-board is the remaining candidate.