Troubleshooting Sinumerik 840C Alarm 20190 NC MD Parameterization

1. Problem Definition

The Sinumerik 840C is a Siemens CNC platform released in the late 1980s and used widely through the mid-1990s on gear-cutting, grinding, and special-purpose machines. On a Gleason Hurth SRS 405 shaving cutter grinder, the controller raises alarm 20190 Parameterization error NC MD on cold start. The alarm indicates that one or more machine data (MD) values the NC read from non-volatile storage on the MMC (Man Machine Communication) PC are out of range, internally inconsistent, or have been corrupted. Because the 840C will not enable any axis or spindle while this alarm is active, the machine is fully down.

This article consolidates the field-proven recovery sequence for the 20190 alarm, the related 300714 (power section code) and 13401 (axis enable) faults that typically appear alongside it on transferred machines, and the special considerations that apply when the original machine builder — here Gleason Hurth — has installed a customer-specific MD set that does not match Siemens defaults.

2. Affected Hardware and Configuration

3. Alarm 20190 — Technical Definition

Alarm 20190 is a generic parameterization error in NC machine data. The 840C internally evaluates MD values in a fixed sequence on each NCK (NC kernel) boot:

1. Load NCMAC.COM and the boot block from the boot partition.
2. Read the active MD image from \NCK\MD\MDxxxxx.DAT.
3. Validate MD against min/max ranges and inter-MD consistency rules.
4. Hand off the validated set to the drive initialization routine.

If step 3 fails, the NCK raises 20190 and writes the offending MD number(s) into the alarm extension. To read the extension on an OP030/OP031:

1. Press the ALARM key on the OP.
2. Select line 20190.
3. Press the DETAIL softkey (or HELP on later MMC versions) to display the failing MD number.

Common failing MD groups on gear-grinding 840C installations:

4. Related Alarms on the SRS 405

Field experience on transferred 840C machines shows 20190 rarely appears in isolation. The two alarms in the source case are typical:

4.1 Alarm 300714 — Power Section Code

On the 840C this corresponds to a SIMODRIVE 611A power section coding plug mismatch. Each 6SN1122 / 6SN1123 power module carries a 10-position DIL switch or coding plug that declares its current rating. The drive firmware compares the plug reading to the value stored in MD 1xxx (drive configuration MD) at power-up. Mismatch = 300714.

Clear procedure:

1. Power off the cabinet, wait 5 minutes for DC bus discharge.
2. Verify each 611A module's coding plug against the SRS 405 wiring diagram (e.g., 8A → 8A, 25A → 25A).
3. If a module was replaced with a different rating, either install the correct plug or update the corresponding drive MD to match the new module.

4.2 Alarm 13401 — Axis Enable / Command Channel

13401 indicates the NC has commanded an axis to move but the controller-side enable chain is open. On the 840C this usually means the controller enable, drive enable, or axis-specific feed/rapid stop signal in PLC interface DB31..DB38 is missing. On the SRS 405 the signal "Achse freigegeben" (axis released) is gated by the Gleason Hurth PLC program, which requires:

• Reference point acknowledgement for all axes (DB31-R1.4 bit, etc.).
• Hydraulic pressure OK (group signal).
• Door interlocks closed.
• Spindle encoder OK.

If 13401 persists after pressing the acknowledge key and the signal shows green on the diagnostics screen but no axis moves, suspect PLC-side enable logic — most often a corrupted @SPF or .SPF PLC program file on the HDD.

5. Root Cause Analysis on a Transferred Machine

When a 840C is disassembled, moved, and re-energized at a new site, alarm 20190 typically has one of the following root causes — in order of frequency observed in field service:

6. Pre-Recovery Checklist

Before clearing the alarm, capture the current state so you can roll back if a step makes things worse:

1. Connect a service laptop to the MMC PC (VGA + keyboard + DOS-bootable 3.5" disk if the original).
2. Copy the entire \NCK, \PLC, and \BACKUP directories to a USB-attached DOS-readable drive.
3. Use SETMD or the service screen to dump the current MD image to a printer or serial port.
4. Document the battery-backup voltage on the NCU (nominal 3.6 V lithium). If < 3.0 V, expect MD loss on next power-down.

7. Step-by-Step Recovery Procedure

Step 1 — Identify the Failing MD

Read the alarm extension as described in Section 3. Note the MD number and current value. Example extension text: 20190 MD 3220 GRRAX1 = 0 — meaning main-spindle gear-stage-1 encoder PPR is 0, which is below the legal minimum.

Step 2 — Enter Machine Builder Menu

1. Press MENU SELECT → SERVICE.
2. Enter the Gleason Hurth service password (machine-specific; request from Gleason).
3. Navigate to MD DISPLAY → AXIS/SPINDLE.

Step 3 — Compare Current MD to Backup

From the DOS prompt on the MMC:

C:> DIR C:\BACKUP\*.DAT /O-D
C:> TYPE C:\BACKUP\MD200105.DAT | MORE

Identify the most recent backup that originated on this exact SRS 405 serial number. The matching serial is typically embedded in the filename header.

Step 4 — Load Backup

1. From the SERVICE screen, select MD LOAD → FROM FILE.
2. Choose the backup file identified in step 3.
3. Confirm with the softkey YES.
4. Wait for the <MD LOAD COMPLETE> banner — typically 60-180 seconds for a full MD set.

Step 5 — NCK Warm Restart

1. Press RESET on the OP.
2. Hold SHIFT + RESET for 3 seconds to force a warm restart.
3. Observe the boot screen for "MD CHECK OK" or a new 20190 with a different extension.

Step 6 — Re-reference All Axes

If 20190 clears but 13401 remains, drive the machine to reference each axis in sequence:

1. Mode switch to JOG or REF.
2. Select axis X, press +, axis travels to reference cam.
3. Repeat for Y, Z, and any rotary axes (A, B, C) on the SRS 405.

The reference acknowledgement bit must be set for every axis before the controller will release the spindle enable.

Step 7 — Confirm Spindle Encoder

For the main spindle, run a short M3 S100 M19 and verify the position readback in the SPINDLE diagnostics screen stabilizes within ±0.1°. If not, the spindle encoder zero offset stored in MD 3xxx was lost — re-teach it via MD SET in the SERVICE menu with the spindle mechanically indexed to a known reference notch.

8. Backup Loading — Best Practices

9. HDD Replacement Procedure

When the original MFM or early-IDE HDD is suspected of media degradation, replacement with a reconditioned or CF-IDE adapter is acceptable:

1. Clone the original HDD sector-by-sector using a DOS tool such as DISKCOPY or HDClone onto a same-or-larger drive.
2. Install the clone, power up, verify boot screen reports the original serial and SW version.
3. If cloning is not possible (old drive dead), install a blank HDD, boot from a 840C installation floppy set, and reload MD + PLC from backup per Section 7.

10. Machine Builder-Specific Configuration

The SRS 405 uses a customer-defined MD set installed by Gleason Hurth. Field evidence from multiple units shows:

• R parameters in the range R0-R99 are reserved for Gleason Hurth internal calculations and are not user-editable.
• Compile cycles for shaving-cutter dressing, wheel profiling, and tooth-by-tooth indexing are loaded via the machine builder menu and stored in MD 7xxx.
• Some axis-pair MD blocks define synchronous following between the dresser and the workpiece spindle. The 20190 alarm frequently points to one of these blocks because their dependency chains are long and sensitive.

Gleason Hurth (now part of Gleason Corporation) still supports many legacy machines via their German service organization. When MD and backup recovery fails, contact Gleason Service with the machine serial number and the SW version shown on the boot screen — typically SW x.y on the line below the 840C banner.

11. Verification Checklist After Recovery

1. No alarm present on the ALARM screen.
2. All axes referenced and the REF LED on the OP is lit for each.
3. Main spindle runs forward and reverse via M3 / M4 at multiple S-values.
4. Tool-change cycle (if present) completes without 13401.
5. One full dry-run of the gear-shaving cycle in single-block mode, watching the ALARM screen throughout.
6. Save the now-working configuration as a new dated backup on at least two media.

12. Quick-Reference Troubleshooting Matrix

13. Long-Term Recommendations

• Maintain a small stock of known-good 840C NCU batteries, 611A coding plugs, and a verified CF-IDE adapter pre-loaded with the machine's MD/PLC image.
• Keep a printed copy of the most recent MD dump and the machine wiring diagram in the cabinet.
• Schedule a quarterly backup-and-verify cycle even when the machine is running faultlessly; bit rot on 1990s MFM/IDE media is common.
• For higher availability, evaluate a bridge upgrade to a Sinumerik 840D sl or 828D with a Gleason Hurth cycle migration kit — though most sites retain the 840C as long as spare parts hold out.