SINUMERIK 840C Alarm 11 — Undervoltage on Secondary Side: Field Diagnostic and Recovery Procedure
Alarm 11 — "Undervoltage on secondary side" is one of the most disruptive faults reported on legacy SINUMERIK 840C turning and milling controls. It appears at every POWER ON, removes NC-Ready, drives the integrated SIMATIC S5 PLC into a safe-state shutdown, and — by control design — wipes user data, forcing an obligatory re-installation of the NC and PLC archives before the machine can be returned to production. Field experience on 840C retrofits consistently attributes the alarm to one of three root causes: (1) a failing primary power supply, (2) a load on the 5 V or 15 V rail shorting through the backplane or a peripheral module, or (3) a wiring/harness fault that perturbs the secondary rails before they settle into tolerance at first contact.
Alarm Definition and Message Attributes
Alarm 11 is displayed on the operator panel in the standard SINUMERIK alarm line, prefixed by the alarm number and qualified by the cancellation criteria.
| Attribute | Value |
|---|---|
| Alarm Number | 11 |
| Alarm Text | Undervoltage on secondary side |
| Reaction | NC-Ready removed; shutdown routine activated to reach a safe state |
| Acknowledgement | POWER ON, or PLC General Reset / End General Reset |
| Appearance | Cyclic at every start-up until the underlying fault is cleared |
| Data Consequence | Obligatory re-installation (NC + PLC archive reload) |
| OEM Customization | Alarm texts modifiable through OEMFORM 6 |
The ability to customize Alarm 11 text via the OEMFORM 6 alarm-dialog tool is documented in the official OEMFORM 6 manual (PDF). This is relevant for OEMs rebranding the control but does not modify the underlying 5 V / 15 V monitoring logic of the Central Services Board.
Power Architecture of the 840C Secondary Side
The SINUMERIK 840C uses a multi-rail power supply seated at the leftmost slot of the sub-rack. The PSU converts the incoming mains or 24 V DC (machine-dependent) into the secondary rails distributed through the backplane to every module:
| Rail | Nominal | Tolerance (typical) | Loads |
|---|---|---|---|
| +5 V | +5.00 V | +4.75 V … +5.25 V | All digital logic on CSB, NC CPU, MMC CPU, PLC CPU, I/O |
| +15 V | +15.00 V | +14.25 V … +15.75 V | Analog Driver Board reference, encoder excitation |
| −15 V | −15.00 V | −14.25 V … −15.75 V | Analog Driver Board bipolar I/O |
| +24 V | +24.00 V | +18 V … +30 V (logic-side input) | Field I/O opto-couplers |
The CSB (Central Services Board) is the supervisory module that monitors these rails. If the CSB detects that either the +5 V or +15 V rail falls below the undervoltage threshold during the boot window, it raises Alarm 11 and refuses to release NC-Ready.
Central Services Board (CSB) and LED H1
The CSB carries a single dual-color LED designated H1. Its state is the most useful first-pass indicator before any board is pulled from the rack.
| LED H1 state | Meaning | Action |
|---|---|---|
| GREEN, solid | +5 V and +15 V rails within tolerance; CSB passes self-test | Suspect transient, wiring, or load-induced dip; proceed to rail measurement |
| RED, solid | +5 V or +15 V undervoltage detected | Alarm 11 active; PSU or load is the prime suspect |
| Off (no light) | Either rail collapsed or CSB itself is unpowered | Verify incoming 24 V / DC-link; swap PSU first |
| Flashing GREEN/RED | CSB self-test in progress (≈ 3 s after power-up) | Wait — do not pull boards during self-test |
If H1 remains RED after the 3-second self-test window, the CSB is reporting that one of the rails never reached regulation or fell out of regulation during boot. If H1 never lights at all, treat the situation as "rail shorted at first contact" and lift one end of the load immediately.
Root Cause Matrix
| # | Fault Domain | Specific Cause | Typical Signature | Difficulty |
|---|---|---|---|---|
| 1 | PSU primary | Aged electrolytics, dried thermal compound, blown input fuse | H1 stays RED even with all boards removed | Low — replace PSU |
| 2 | Backplane / connector | Oxidized DIN-96-pin socket, foreign debris in slot | Alarm clears after reseating CSB only | Low — clean, reseat |
| 3 | Analog Driver Board | Output-amplifier short pulling +15 V rail down | Alarm appears only with axis driver populated | Medium — board swap |
| 4 | MMC CPU Board | Voltage-regulator failure on HMI/operator chain | Display dark, H1 RED, NC-CPU LED off | Medium — board swap |
| 5 | CPU Interface Card (PLC) | Internal short on the +5 V backplane tap | Alarm appears with PLC card populated only | Medium — board swap |
| 6 | NC CPU Board (6FC5110-0BA01-1AA0) | Logic-die short, decoupling-cap failure | Alarm persists after every other board is removed | High — NC CPU swap + reload |
| 7 | Wiring / harness | Pin-to-pin short at first insertion (cold-start) | Alarm present on initial commissioning only | Low — examine harness |
Items 1 and 7 (PSU and wiring) are the most common field-reported root causes on turning machines whose 840C has been in service for more than a decade. Items 4–6 require a methodical board-isolation procedure.
Preconditions Before You Open the Cabinet
Required tooling and prerequisites:
- Archive of NC and PLC data captured with the SINUMERIK 840C service tool (PG 720, PG 740, or later Siemens programming device with the 840C toolbox). This archive is consumed during the obligatory re-installation.
- ESD wrist strap bonded to the cabinet PE rail.
- Digital multimeter with 0.05 % DC accuracy on the 5 V range (Fluke 87V, Gossen M205, or equivalent).
- Spares available on site: PSU, CSB, Analog Driver Board, MMC CPU Board, CPU Interface Card (PLC) — at minimum the PSU and the CSB.
- Documentation binder or PDF set of the SINUMERIK 840C Diagnostics Manual and Operator's Manual available on Siemens Industry Online Support.
Diagnostic Procedure — Step by Step
- Power up and observe LED H1. Record whether H1 is off, red, or green. Note whether the alarm appears immediately or after a delay. Cyclic alarm recurrence at every POWER ON means the fault is structural, not transient.
- Measure the secondary rails directly. With the rack fully assembled, place the meter on TP5 (GND) and TP1 (+5 V) of the CSB; record +5.00 V ± 2.5 %. Repeat on TP2 (+15 V) and TP3 (−15 V). Any rail more than 5 % out of nominal forces a "PSU first" replacement.
- Remove non-essential boards. With the cabinet de-energized and ESD-bonded, pull the MMC CPU Board, the Analog Driver Board, and the CPU Interface Card (PLC), leaving only the CSB and the PSU populated.
- Re-energize and re-check H1. If H1 is now GREEN, the fault lives on one of the removed boards. Proceed to swap them in, one at a time, until H1 returns to RED — that board is the load culprit.
- If H1 is still RED with only CSB + PSU in the rack, the backplane or PSU is at fault. Reseat the CSB on a clean edge-connector; if the fault persists, replace the PSU before condemning the CSB itself (PSUs are cheaper and fail more often).
- Verify the cold-start wiring. The OEM documentation explicitly flags Alarm 11 as a possible false-positive on initial start-up caused by wiring transients. Inspect every new or recently disturbed cable; pay particular attention to encoder cables routed parallel to the 24 V drive supply.
Board Isolation Flowchart
The flowchart below illustrates the sequential board-isolation procedure. Start at the top, execute each step until H1 turns GREEN; the board last inserted is the failed component.
Replaceable FRUs and Reference Designators
| FRU | Siemens Order Number (range) | Removal Order |
|---|---|---|
| Power supply unit | 6FC5 252-… series | 1st (suspect most) |
| Central Services Board (CSB) | 6FC5 110-0… series | 2nd |
| NC CPU Board | 6FC5110-0BA01-1AA0 (confirmed) | 3rd — confirm by isolation |
| MMC CPU Board | 6FC5 110-0… (HMI-side) | 4th |
| Analog Driver Board | 6FC5 110-0… (axis-side) | 5th |
| CPU Interface Card (PLC) | 6ES5 2xx-x… (S5 submodule) | 6th |
Obligatory Re-installation and PLC General Reset
Once the underlying hardware fault is resolved, the 840C remains in a safe-state halt until two operations are completed:
-
PLC General Reset. Engaged via the operator-panel key sequence:
General Resetsoft-key, thenEnd General Reset. This clears PLC data, resets the S5 work flags, and clears Alarm 11 from the buffer. - NC archive re-installation. Performed via the SINUMERIK 840C service tool or a PG connected to the MMC CPU. Load the previously archived NC data set, then the PLC source (S5 STL or LAD). After both archives are restored, Alarm 11 is permanently cleared and the channel can be returned to AUTO.
If Alarm 11 returns immediately after the re-installation, the load-side fault was not eliminated — return to Diagnostic Procedure step 4. Do not loop the reset → reload cycle indefinitely without re-validating the rails.
Verification and Post-Repair Commissioning
| Test | Expected Result | Acceptance |
|---|---|---|
| LED H1 steady GREEN, 5 s after POWER ON | CSB has confirmed both +5 V and +15 V | Mandatory |
| +5 V at CSB TP1 | +4.95 V … +5.05 V | Mandatory |
| +15 V at CSB TP2 | +14.85 V … +15.15 V | Mandatory |
| Alarm 11 not present in the alarm history after three cold starts | Cold-start consistently clean | Mandatory |
| Axis reference traverse > 50 mm in each axis | No shadow alarm | Recommended |
| PLC cyclic 50 ms scan | Scan time within S5 project tolerance | Recommended |
Related and Frequently Co-Occurring Alarms
| Alarm # | Text | Relation to Alarm 11 |
|---|---|---|
| 12 | Overvoltage on secondary side | Companion fault; usually rail damage or load dump |
| 13 | Undervoltage on primary side | Check 24 V DC-link to PSU first |
| 1xx series | NC CPU / SERCOS / drive-link faults | Often follow Alarm 11 if the NC reboots during the undervoltage window |
| 6xxx series | PLC S5 fault | Generated when PLC enters the safe state triggered by the secondary-side alarm |
Field Notes and Prevention
- Capture the latest NC and PLC archives to a PG before any reset attempt — the obligatory re-installation consumes in-RAM data that is not recoverable after PLC General Reset.
- On controls that have been in service for 10+ years, preemptive PSU replacement during scheduled maintenance is cheaper than an Alarm-11-driven return-to-service event.
- When retrofitting a turning machine, route the 24 V DC link to the PSU from a dedicated regulated supply; sharing the cabinet with motor contactors is a known contributor to cold-start glitches on the +15 V rail.
- Apply OEM alarm-text overrides only through the OEMFORM 6 toolchain — unauthorized binary patches to the alarm text are not supported and may mask a genuine Alarm 11 in service.
- Capture an active
LED H1photo on every cold start and store with the machine logbook — a pattern of H1 flicker at first contact is a leading indicator of a developing PSU rail.
FAQ
What does "secondary side" mean in Alarm 11?
The 840C PSU is galvanically isolated. The "primary side" is the incoming mains or 24 V stage; the "secondary side" is the regulated output stage (+5 V, +15 V, −15 V) that feeds the backplane. Alarm 11 is raised by the CSB when the secondary rails fall below their undervoltage threshold during the boot window.
Will a POWER ON reset alone clear Alarm 11?
On a hardware-healthy rack, POWER ON will clear the alarm because the rails will come up within tolerance. When the alarm is cyclic (re-appearing on every POWER ON), the underlying fault is still present and the reset only re-triggers the shutdown routine — a PLC General Reset and re-installation will be required once the hardware is repaired.
Is data loss guaranteed after Alarm 11?
The CSB forces the 840C into its safe-state shutdown, which by design triggers obligatory re-installation. Always back up the NC and PLC archives before any reset attempt. After repair, reload the archive rather than expecting the original RAM contents to survive.
Which board is the NC CPU reference 6FC5110-0BA01-1AA0?
It is the SINUMERIK 840C NC CPU (Numerical-Control processor) module seated in the sub-rack. It hosts the NC kernel and is the highest-confidence board to suspect only after the PSU and the CSB have been eliminated as fault sources via the isolation procedure above.
Can Alarm 11 appear during initial commissioning only?
Yes. The OEM documentation explicitly notes that Alarm 11 can be raised during initial start-up without a true fault being present, due to harness transients at first contact. Re-seat the connectors and re-POWER ON; if the alarm clears and does not return, no further action is needed.