SINUMERIK 840D SHOPTURN Resolving 'Waiting for PLC Connection'

1. Problem Overview

The SINUMERIK 840D SHOPTURN controller on a DMG MORI CTX310 lathe boots to the splash logo and stops. Pressing any key displays HMI Embedded version 06.02.42 followed by waiting for plc connection on the operator panel, and the system hangs indefinitely. Two additional symptoms almost always accompany this fault:

• No drive recognition — the HMI never lists the SIMODRIVE 611 or SINAMICS S120 modules in the drive status page.
• BIOS date frozen at 1999 (typically 01/01/1999 or 06/01/1999) on the operator panel front-end.

The combination of these three symptoms points to a deeper failure than a missing PLC program: the HMI Embedded cannot establish the internal handshake to the NCU/PLC, and the operator panel PC (PCU or HMI Embedded target) has lost its RTC backup. In shop-floor practice this is one of the most common field returns on legacy 840D powerline and 840D sl controllers that have sat un-powered through storage or a long shutdown.

2. Affected Hardware and Firmware

The fault pattern is documented on the following configurations:

Reference the official SINUMERIK 840D sl Operating Manual and the SINUMERIK PCU Base Software IM4–IM8 documentation for software compatibility matrices. HMI Embedded 06.02.42 is the legacy SHOPTURN deliverable that predates the SINUMERIK Operate line; it must be paired with PCU 50.5 / 50.3 or an HMI Embedded target as listed in the manual.

3. Root Cause Analysis

Three failure domains converge when this fault is reported. Treat all three simultaneously — fixing only one will leave the machine dead.

3.1 PLC connection timeout

HMI Embedded must handshake with the PLC over the internal MPI/PROFIBUS link (baud rate 187.5 kbit/s default) within the boot window. If the NCU fails to publish its PLC online status, the HMI sits at waiting for plc connection. Common triggers:

• Empty or corrupted PLC user program on the CompactFlash card (NCU CF slot).
• PLC firmware stop (STOP LED solid on the PLC317/PLC319 of the NCU).
• Corrupted NCU boot files (\mmc2\..., \user\...) — typically after a battery-fail or partial write.
• Handshake DB missing (DB7 / DB9 on default 840D PLC programs).

3.2 Drive recognition failure

Drives are not enumerated independently of the PLC — HMI Embedded reads the drive list from the same NCU buffer that the PLC populates through OB1 / OB100 startup. If the PLC handshake never completes, the HMI never receives the drive list, even if the drives themselves are healthy. Verify drive health separately with the SIMODRIVE 611D / SINAMICS S120 seven-segment display before assuming drive hardware failure.

3.3 BIOS date 1999 — dead RTC battery

A CMOS battery (typically BR2032, 3 V, 190 mAh) on the operator panel PC keeps the RTC and CMOS settings alive while mains is removed. Once the cell drops below ~2.5 V the BIOS resets to defaults and the clock returns to 01/01/1999 (or the last default year programmed). Symptomatic effects:

• BIOS defaults loaded (boot order, COM ports, integrated peripherals).
• RTC returns 1999, causing Windows / HMI Embedded license checks, file timestamps, and certificate validations to fail.
• On HMI Embedded targets, an RTC fault can prevent the HMI service from completing initialization — the HMI stays at the splash until the cell is replaced.

The dead-cell hypothesis is strongly supported by an observed battery voltage drop, bulging cell, or white residue on the holder. Replace the cell first, because every other recovery step depends on a stable RTC for log timestamps and license verification.

4. Diagnostic Procedure

1. Verify mains and 24 VDC supply. Measure +24 V at NCU X111 terminal. Tolerance: 20.4 V – 28.8 V. Below 22 V the NCU can complete POST but fail the PLC handshake.
2. Confirm NCU LED status. On a healthy NCU 561/571/573 you should see: PWR (green), SF (red) momentarily then OFF, BF (red) OFF or briefly lit, CF active during CF access. A solid red SF indicates PLC stop — proceed to STEP 7 / TIA Portal diagnostics.
3. Inspect the operator panel PC LEDs. PCU 50 HDD LED should pulse, not stay dark. A dark HDD LED indicates the CompactFlash boot media is unreadable.
4. Measure the CMOS cell. With the panel powered down, lift the cell and read it under no-load. A reading below 2.7 V means replace. Always retest after a 60-second rest before condemning the cell.
5. Read NCU diagnostic buffer. Connect a PG / laptop running SIMATIC Manager (STEP 7 V5.5 or TIA Portal V16/V17) over Ethernet or MPI. Online → Accessible Nodes → select NCU → Module Information → Diagnostic Buffer. Note the most recent PLC stop events and any OB100 / OB102 startup OB failures.
6. Check CF card contents. Remove the NCU CF card (with power removed) and read it in a PC using a CompactFlash reader. Verify the presence of mmc2 directory tree and the user's PLC archive. The SHOPTURN project typically lives at D:\user\sinumerik\... on the PCU side and /card/user/... on the NCU side.

5. Mandatory Backup Before Recovery

1. From the HMI Embedded menu (if accessible) or via Ethernet using SINUMERIK Toolbox / WinSCP / RCS Commander, run a Series Start-up Backup. Store on an external USB stick or network share.
2. Back up the NCU CF card image to a PC with dd or the HMI service menu — the entire card is required, not just the mmc2 tree.
3. Export the PLC project from STEP 7 / TIA Portal (Archive → S7 Program → Archived User Program, “.zip” or “.awl”).
4. Back up drive parameters via SimoCom U / STARTER — export each SIMODRIVE 611D drive as *.dnx and each SINAMICS S120 as *.bak.

6. Step-by-Step Recovery

Step 1 — Replace the CMOS battery

1. Power down, lock out, wait 5 minutes for DC-link discharge.
2. Open the operator panel rear cover; locate the CR2032 / BR2032 holder on the PCU 50 mainboard or the HMI Embedded target SBC.
3. Note polarity (positive facing up on most PCU 50 boards). Replace with a fresh cell.
4. Reapply power; enter BIOS (typically DEL or F2), set date/time, save and exit. Verify the clock holds after a mains cycle.

Step 2 — Restore the NCU boot image

1. If the CF card is empty or the directory structure is missing, rewrite the NCU image from a known-good backup using the procedure in the SINUMERIK CF Card Imaging Tool documentation.
2. Power up the NCU and verify POST completes without SF/BF errors.

Step 3 — Clear and reload the PLC

1. Connect to the NCU from STEP 7 / TIA Portal.
2. Open the project (or import the archived program), then perform PLC → Clear/Reset with the “Reset memory and retain variables” option unchecked initially.
3. Perform a full download (blocks, system data, hardware configuration). Observe the diagnostics buffer for download errors.
4. Place the CPU in RUN. The RUN LED should be steady green; STOP LED off.

Step 4 — Re-establish HMI–PLC handshake

1. Restart the HMI Embedded service from the operator panel service menu, or cycle mains.
2. Watch the splash. The waiting for plc connection prompt must transition to the SHOPTURN main screen within ~90 seconds on a healthy system.
3. If the prompt persists, re-verify the NCU ↔ PCU MPI/PROFIBUS cable. The standard link uses a 9-pin D-sub on the PCU side and a Siemens MPI cable (6FX2002-1AA01-0xx). Replace if pins are corroded or the shield is broken.

Step 5 — Drive commissioning

1. Once the PLC is in RUN, drives will be enumerated automatically by the SHOPTURN PLC program.
2. Confirm on the HMI — the drive status page should list each SIMODRIVE 611D axis (X1, Z1, spindle, etc.) and any SINAMICS S120 modules with their drive numbers.
3. Cross-check seven-segment displays: SIMODRIVE 611D shows a steady status (e.g., 0 = ready, 7 = wait for enable). SINAMICS S120 shows S or r when commissioned and waiting for enable.
4. If drives do not appear, run a PROFIBUS scan (PG → Accessible Nodes) to verify all nodes respond. Common failure points: terminator switches on the last drive, PROFIBUS address conflicts, broken shield termination at the 9-pin D-sub.

7. Verification Checklist

8. Fault & Alarm Reference

9. Preventive Maintenance

• CMOS cell replacement schedule: every 5 years or at first observed date drift. Document the cell lot number on the panel rear.
• Annual backup: a full series start-up backup on the SHOPTURN controller and a separate NCU CF card image. Store at least two generations off-machine.
• Battery health log: record cell voltage during PM. Replace any cell reading < 2.9 V under load.
• PROFIBUS annual check: re-torque D-sub connectors, verify shield termination, confirm 120 Ω terminators on first and last nodes.
• HMI Embedded firmware pinning: keep at 06.02.42 unless the machine OEM (DMG MORI) supplies a tested patch. Avoid drifting to SINUMERIK Operate without explicit approval — the SHOPTURN HMI is hardware-locked to PCU 50.

10. Field-Proven Notes

11. When to Escalate to Siemens Support

If after replacing the CMOS cell, restoring a known-good NCU image, clearing and reloading the PLC, and verifying PROFIBUS cabling the waiting for plc connection prompt persists and drives remain absent, escalate to Siemens Industry Online Support with the following attachments:

• NCU diagnostic buffer export (.txt).
• PLC project archive (.zip) and current hardware configuration.
• Series start-up backup of the HMI Embedded.
• Photograph of the NCU front (showing all seven-segment displays and LEDs).
• Photograph of the PCU 50 / HMI Embedded rear showing the BIOS date and battery holder.
• Drive seven-segment codes and PROFIBUS node addresses.

Reference the SINUMERIK 840D sl PCU Base Software IM4–IM8 and the SINUMERIK 840D sl Operating Manual when filing. Quoting the exact NCU part number (e.g., 6FC5356-0BB11-0AA0) and firmware version (CF card image build) accelerates triage.