SINUMERIK 3M 03840 Board Reset Failure: Resolving Two Red LEDs After Battery Loss

1. Problem Summary

Following a backup battery swap on the SINUMERIK 3M power supply (carrying the ULA 1U038E-3 supervisory processor), the machine operator or service engineer must execute a documented cold-start reset. When the procedure is performed — even repeatedly, with the green status LED on the 3M controller eventually coming on after 20 to 30 attempts — the 03840 board (6FX1122-2AB02) continues to display two red LEDs instead of the single red LED that indicates a normal post-load standby state.

The visible symptoms reported in the field are:

• 3M controller: green LED (eventually, after repeated attempts)
• 03840 board: two red LEDs continuously lit, left LED no longer flashing
• Keyboard: unresponsive, no keystrokes registered
• Screen: blank, no boot text, no menu tree
• Machine: dead — no axis enable, no spindle enable, no program execution possible

The intended post-reset state is:

• 3M controller: green LED (steady)
• 03840 board: right red LED only; left red LED extinguished
• Keyboard: responding to operator input
• Screen: boot text visible, file/parameter load indicator progressing

2. Hardware Identification: 03840 / 6FX1122-2AB02 Coupling Card

The card designated "03840" in the SINUMERIK 3M service documentation is the Siemens order number 6FX1122-2AB02. Its function is the NC-to-PLC coupling — it carries the parallel handshake and data bus between the numerical control kernel (NC-CPU) and the programmable logic controller (PLC-CPU). Both CPUs are implemented on the same backplane, with the coupling card bridging their address, data, and interrupt lines.

Reference the official Siemens Industry Online Support portal for current spare-part availability and historical part cross-references:

• Siemens Industry Online Support — Global Entry
• Siemens Industry Mall — 6FX1122-2AB02 Product Page (legacy/archived listing)

3. LED Status Reference

The 03840 board carries two red LEDs, oriented left and right when the card is installed vertically in the SINUMERIK rack. The two LEDs do not have identical meaning and must be interpreted as a pair. The transitions described in the source — left LED flashing during bootstrap, then stopping flashing and latching on while the right LED remains on — map directly to a coupling-handshake failure state.

The reported fault — left red LED stops flashing and remains on, both red LEDs illuminated — corresponds to the FAULT row. The left LED has transitioned from its self-test / load phase to a latched fault state, indicating that the bootstrap process did not converge.

4. Backup Battery Replacement Precautions

SINUMERIK 3M retains the machine parameters, tool tables, part programs, and PLC user program in battery-buffered CMOS static RAM. The backup cell is housed in the power supply module and is supervised by the ULA 1U038E-3 processor. The ULA monitors the battery rail and generates a "battery low" warning to the NC kernel before the cell voltage drops below the SRAM retention threshold.

Precautions that must be observed before, during, and after battery replacement:

1. Connect the machine to a stable mains supply; do not perform the swap on a UPS that will time out within the swap interval.
2. Verify with a multimeter that the replacement cell measures within its nominal voltage on load (10 mA test load) before installation. A new cell out of specification can produce the same fault state as a depleted cell.
3. Complete the mechanical swap as quickly as the connector and holder allow. SRAM retention time after V_bat removal is limited by the on-board buffer capacitor on the RAM board; beyond this window, parameter loss is certain.
4. If the swap interval is exceeded, all parameters must be reloaded from the machine's archived parameter sheet (typically a printed listing kept in the electrical cabinet door pocket) and the cold-start reset procedure must be executed from a known-good parameter set.
5. After cell insertion, do not power-cycle the machine for a stabilization period — the buffer capacitor must charge to a stable retention voltage before any reset is attempted. A 5–10 minute dwell is field-recommended.

5. Cold-Start Reset Procedure

The cold-start reset procedure for the SINUMERIK 3M family is documented in the Siemens service manual set for System 3 (publication series 6FX1 100). The procedure below is the sequence the source user is following, with additional timing and verification constraints added based on the observed symptoms.

5.1 Pre-conditions

• Machine mains power ON, all three-phase contactors closed.
• 24 VDC logic rail present at the SINUMERIK rack backplane (verify with multimeter at the test points on the power supply front panel).
• Backup battery installed and verified (see Section 4).
• 03840 board firmly seated in its slot; both DIN 41612 connectors fully engaged.
• All other SINUMERIK 3M sub-assemblies (NC-CPU, PLC-CPU, operator panel I/O, servo I/O) present and seated.

5.2 Switch and button inventory

The reset procedure uses the following controls. The exact labels vary by machine builder, but the Siemens service documentation uses the labels below:

5.3 Execution sequence

1. Set the RUN/STOP switch on the 3M controller to STOP.
2. Press and hold simultaneously: Reset (3M), Restart (3M), and Reset (PS). Hold for a minimum of 3 seconds. The 3M controller's red LED will illuminate; the 03840 board's left red LED will begin to flash.
3. Release all three controls in a defined order with 1-second dwell between releases — do not release all three simultaneously. A safe order is: Restart (3M) first, then Reset (3M), then Reset (PS).
4. Move the RUN/STOP switch from STOP to RUN within 2 seconds of releasing the last control.
5. Press and release Reset (PS) a second time. This is the "acknowledge cold-start" pulse that the ULA 1U038E-3 uses to commit the request.
6. Observe the 03840 board: the left red LED should continue flashing for 5–30 seconds as the bootstrap progresses, then extinguish. The right red LED should illuminate and remain steady. The 3M controller's green LED should illuminate within the same window.
7. If after 60 seconds the left red LED has not extinguished, the reset has failed. Return to step 1 and repeat, or proceed to the diagnostic matrix (Section 9).

6. 03840 Board Configuration Switch Position

The 03840 board carries a single configuration switch on the top edge of the printed circuit board. This switch selects the address and operating mode of the coupling card within the SINUMERIK 3M backplane. The switch is typically a multi-position DIP or slide switch, depending on the card revision, and the silk-screen legend printed on the PCB is the authoritative reference for the installed board.

For the cold-start reset procedure, the switch must be in the position designated "Normal" or "RUN" in the silk-screen legend. The exact label varies by card revision; the table below uses the most common field-observed labels. Verify against the actual silk-screen on the installed card before applying.

The configuration switch position must be confirmed before the cold-start reset is attempted and before the 03840 board is reseated. A switch in the Test (T) or OFF (0) position will cause the coupling handshake to fail on every reset attempt, regardless of how many times the procedure is repeated, and will produce the two-red-LED fault state.

7. Reset Sequence State Machine

The reset procedure can be modelled as a deterministic finite state machine. Each state corresponds to a specific combination of LEDs on the 3M controller and the 03840 board, plus the keyboard and screen response. The state machine makes explicit which transitions are valid and which lead to the latched FAULT state.

Figure 1 — Cold-start reset state machine. The path S0 → S1 → S2 → S3 → S4 → S5 represents a successful reset. The path S4 → S6 is the fault path; recovery requires diagnostic action (D1, D2, or D3) followed by a return to S1.

8. Root Cause Analysis

The reported symptom — 3M green LED on, 03840 left red LED on (no longer flashing), 03840 right red LED on, keyboard dead, screen blank — is consistent with the NC-CPU having booted successfully but the NC-to-PLC coupling handshake having failed. The most probable root causes, ranked by frequency of occurrence in field service:

1. Incorrect switch position on the 03840 board. A configuration switch in the Test or OFF position will prevent the coupling card from responding to the cold-start acknowledge. This is the most common cause and is the first item to verify.
2. Buffer capacitor not charged. The reset is attempted within seconds of the battery swap, before the on-board buffer capacitor has reached its retention voltage. The ULA 1U038E-3 sees an unstable battery rail and aborts the cold-start sequence mid-acknowledge.
3. Timing error in the release sequence. The three reset controls are released too quickly, in the wrong order, or with insufficient dwell time. The ULA 1U038E-3 does not latch the cold-start request and the NC-CPU returns to normal operation without having triggered the parameter reload.
4. Backup battery cell out of specification. A replacement cell measuring below its nominal voltage on load will not sustain SRAM retention. The ULA 1U038E-3 detects this on the first battery rail check and aborts the cold-start.
5. Partial parameter corruption. If the battery was depleted for an extended period, the file/parameter memory contains a partially corrupted data set. The cold-start loads the corrupted data, the coupling card's checksum validation fails, and the handshake latches off.
6. 03840 board hardware fault. Coupling card failure (typically a failed bus driver, PROM, or glue-logic IC). Probability is low if the machine was operating correctly before the battery swap, but increases with machine age and exposure to coolant mist or oil contamination.

Cases 1 through 4 are addressable by procedural correction. Case 5 requires a parameter reload from the machine's archived parameter sheet. Case 6 requires card replacement.

9. Diagnostic Verification Matrix

The following matrix maps the observed fault to the most likely root cause and the appropriate verification step. The "Decision column" indicates the order in which verifications should be performed.

10. Hardware Substitution and Sourcing

If all diagnostic steps D1 through D5 pass and the two-red-LED fault persists, the 03840 board should be considered suspect. The 6FX1122-2AB02 is a legacy part and is no longer in current production. Sourcing options for the field engineer:

• Siemens Industry Mall — legacy stock. Some authorized Siemens distributors maintain small stocks of legacy SINUMERIK 3 series cards. Order under MLFB 6FX1122-2AB02 and verify the card revision printed on the component side.
• Industrial surplus dealers. Cards recovered from decommissioned SINUMERIK 3M systems are available from European and North American industrial surplus dealers. Verify the card revision (-AB02, -AB03 or other) matches the machine's documented revision before installation.
• Machine builder service exchange. For DECKEL FP2NC and similar late-1980s machines, the original machine builder (or its successor organizations) may offer a card exchange or test service. This is the lowest-risk path because the exchanged card is typically factory-tested against the same machine configuration.

Reference links for spare-part sourcing and legacy support:

• Siemens Industry Online Support — SINUMERIK Legacy Products
• Siemens Industry Mall — Product Catalog
• SINUMERIK CNC Product Family Overview

11. Frequently Asked Questions