Resolving Siemens 840D Alarm 810005: STEP 7 Diagnosis Guide

Overview of Alarm 810005 on SINUMERIK 840D MMC100.2

Alarm 810005 "Operating states sequence event, error analysis via STEP 7 required" is a PLC-side general alarm raised by the SINUMERIK 840D system firmware when the integrated S7 PLC detects an illegal or unexpected transition in the operating-state sequence of the machine's safety/logic chain. The alarm is presented on the MMC100.2 HMI when the HMI polls the PLC and reads at least one of the eight user alarm bits (DB2.DBB0–DBB7 on the SINUMERIK 840D MMC100.2) set by the user program. Alarm 810005 does not carry a precise NCK fault address; it is a wrapper that points the operator directly at the PLC user program and forces a STEP 7 / SIMATIC S7 diagnostic session to read the S7-300 CPU diagnostic buffer.

The MMC100.2 generation is the predecessor of MMC103 and PCU50 hardware platforms and corresponds to a SINUMERIK 840D (Powerline) system with a SIMATIC S7-300 PLC sub-rack. The relevant Siemens documentation for this platform family is the SINUMERIK 840D sl/840Di sl/828D Operator Components Manual (08/2014 edition, 6FC5397-1CP40-3BA0) and the SINUMERIK 840D sl Diagnostics Manual (06/2019, 6FC5398-6AP40-3BA3). Although the cited manuals target 840D sl hardware, the alarm numbering, the PLC interface, and the diagnostic flow remain identical for the Powerline/MMC100.2 generation. The classic 840D/810D Commissioning Manual is the SINUMERIK 840D/810D Commissioning Manual (Edition 04.96, 6FC5297-1AD10-0AP0) and the SINUMERIK 840D sl/840Di sl/828D Diagnostics Manual (01/2018, 6FC5398-6AP40-3BA0).

On an MMC100.2 panel, alarm 810005 is displayed in the red alarm line on the bottom of the HMI; clearing the alarm by power-cycling the NCU only suppresses the display because the underlying PLC fault is latched in the S7 CPU's diagnostic buffer. The PLC bit that triggers 810005 is M0.0 on the SINUMERIK PLC interface (DB2.DBB0 bit 0) or one of the user-defined PLC alarms DB2.DBB0–DBB7 that the user program has assigned to the parts-catcher and spindle-lock sub-systems. The correct response is therefore not an NC reset but a structured PLC diagnosis with STEP 7.

Alarm 810005 Definition, Class, and Numbering Range

Siemens alarm numbering on the SINUMERIK 840D is partitioned into the ranges listed below. Alarm 810005 falls in the "PLC general alarms" range, which is reserved for user-program-generated messages routed through the PLC→HMI interface.

Within the 810000 range, alarm 810005 is defined in the SINUMERIK Operate / MMC100.2 alarm text database as "Operating states sequence event, error analysis via STEP 7 required". The alarm is always associated with a fault in the PLC user program, not in the NCK. The PLC program has either:

1. Detected an illegal sequence in the state machine (e.g., spindle-lock cylinder requested to engage before the previous disengage cycle completed).
2. Latched a watchdog / OB121 / OB122 fault (programming or access error in a non-recoverable OB).
3. Lost retentive data after a battery-backed SRAM drop on the S7-300 CPU (typical for MMC100.2 systems that have been in service 15+ years).

Symptoms Reported on a Lathe with MMC100.2 V.1.01

The reported machine exhibits the following layered symptoms, all of which are downstream of the same root cause and should be treated as a single fault tree:

1. Alarm 810005 appears at power-on while the machine is idle (not cutting). No NC motion was in progress when the alarm was raised.
2. A power-cycle clears the HMI line momentarily, but secondary PLC alarms for the parts catcher and the spindle locking cylinder appear immediately after the PLC restart. These alarms are user-defined in DB2.DBB4 / DB2.DBB5 in the standard SINUMERIK 840D MMC PLC template (part of the SINUMERIK 840D/810D Commissioning Manual).
3. Manual operation of the hydraulic/pneumatic valves by depressing the small red manual-override button on each valve still actuates the cylinder. This proves that 24 V DC, the proportional valve coil, the limit switches, and the I/O wiring are intact. The fault is upstream of the valve, in the PLC output path or in the PLC program logic that drives the output.
4. The fuse on the customer-supplied picture is the standard MMC100.2 backplane / PLC-output group fuse, 5×20 mm glass, T 6.3 A / 250 V, supplied from the secondary of the 230 V / 24 V DC control transformer mounted inside the operator-panel cabinet.

The combination of "alarm 810005 + downstream PLC user alarms + blowing fuse" is the classic signature of a stuck output / shorted wiring fault combined with loss of PLC retentive data. The PLC user program can no longer drive the outputs correctly because the topology database (which valve is on which output byte) is corrupted, so the outputs default in a state that sinks excessive current through one branch of the valve manifold, blowing the fuse.

Root Cause Analysis Methodology

Begin the diagnosis in the order listed below. Each step is gated by an expected outcome; do not skip ahead.

Step 1 — Read the S7-300 CPU Diagnostic Buffer

The diagnostic buffer (Diagnosepuffer) is a 100-entry ring buffer in non-volatile memory of the S7-300 CPU. It is the single most important data source for 810005. It contains, in chronological order, every OB entry (OB1 cycle time exceptions, OB85, OB86, OB100, OB101, OB102, OB121, OB122), every STOP→RUN transition, every parameter assignment error, and every time-of-day stamp. Open it from STEP 7 via PLC → Diagnostic/Setting → Diagnostic Buffer or by right-clicking the CPU in the online view and selecting Module Information → Diagnostic Buffer.

For the 16-year-old MMC100.2 system described, the most common entries seen before 810005 are:

• STOP because of OB not loaded — the S7-300 CPU was rebooted without the OB86 (rack failure) or OB122 (I/O access error) blocks loaded, causing the CPU to STOP and the NC to raise 810005.
• Battery exhausted, CPU in STOP — the buffer-backed SRAM battery is dead. The S7-300 CPU cannot retain retentive M, T, C, and DB data through power-off. This is by far the most common cause on a 16-year-old 840D.
• I/O access error (OB122) in FBxxx, DBxxx, Byte x, Bit x — the user program is reading or writing an I/O byte that no longer exists (a re-wired input or a removed terminal block).
• Time-of-day interrupt OB10/OB11 lost — a side effect of the battery issue.

Step 2 — Identify Which PLC User Alarm Bits Are Set

In the SINUMERIK 840D MMC100.2 PLC interface, eight bytes of user alarms are exposed to the HMI. The default assignment is shown in the table below; the exact assignment for any given machine tool is defined in the manufacturer-specific PLC project. Read the live value with STEP 7's Monitor / Modify Variable function or with the HMI's PLC → Status menu.

If, for example, the parts-catcher alarm appears as alarm 701600 on the HMI line, the offending bit is DB2.DBB6 bit 0. Write down every bit that is set; this is the error map of the machine.

Step 3 — Cross-Check the Spindle-Lock and Parts-Catcher I/O

Once the diagnostic buffer is read and the bits are mapped, walk the program from the alarm bit back to the output. For the parts catcher, the typical signal chain is:

1. Input: I 0.x "Parts catcher extended" (proximity switch on the cylinder rod).
2. Input: I 0.x "Parts catcher retracted" (proximity switch on the cylinder body).
3. Logic: FC / FB "PartsCatcher" with two latching states (extended / retracted) and a watchdog timer that expects the cylinder to complete the move within 2–3 s.
4. Output: Q 0.x "Parts catcher extend solenoid" — 24 V DC, 2.5 W.
5. Output: Q 0.x "Parts catcher retract solenoid" — 24 V DC, 2.5 W.

If the output byte is stuck high on one channel because the program is latched in an illegal state, the corresponding solenoid is continuously energized. With a 24 V / 2.5 W coil at continuous duty, the coil will draw 104 mA. If the feedback proximity switch is shorted, the input current can rise to 200–300 mA on a single output group. Five valves in continuous duty on the same output byte draw 520 mA, which is within the 6.3 A fuse rating, but if the wiring is shorted to ground (e.g., a pinched cable on the parts-catcher carriage), the fuse blows within 100 ms.

Prerequisites: Hardware and Software Required for Diagnosis

To read the S7-300 diagnostic buffer on a 16-year-old SINUMERIK 840D with MMC100.2, the following are required. None of these is a free download; STEP 7 is licensed software, and Siemens discontinued the parallel-USB / parallel-MPI cable years ago.

STEP 7 Online Connection Procedure (MPI/DP)

The standard procedure to bring STEP 7 online with the S7-300 inside the SINUMERIK 840D MMC100.2 NCU is as follows. The SINUMERIK MMC100.2 has two X122 / X132 PROFIBUS / MPI ports on the back; either can be used, but do not connect to the NCK's PROFIBUS-DP line (X122 on the 840D) because that line carries drives and may interfere with the S7-300's MPI traffic. The 840D's PLC side exposes its own MPI / PROFIBUS at X122 (lower) on the S7-300 CPU itself.

1. Install STEP 7 V5.5 SP4 (or V5.6) on the service laptop. Restart, then accept the license-key prompt and insert the 6ES7810-... USB license stick.
2. Open SIMATIC Manager and create a new project. Right-click the project icon, choose Insert New Object → SIMATIC 300 Station. The hardware is recognized later via PLC → Upload Station to PG.
3. Set the PG/PC interface: Options → Set PG/PC Interface → PC Adapter (MPI) → Properties. Set the baud rate to 187.5 kbps (this is the only baud rate supported by an MMC100.2 system). Set the station address to 0. The S7-300 CPU default MPI address is 2; do not change it unless the system integrator changed it.
4. Connect the PC Adapter USB A2 to the service laptop's USB port, then plug the 9-pin Sub-D connector of the adapter onto the MPI socket of the S7-300 CPU. Tighten the thumb screws.
5. Power the NCU if it is not already powered. From SIMATIC Manager choose PLC → Display Accessible Nodes. The S7-300 CPU should appear with its MPI address (default 2, possibly 1, depending on configuration). If it does not appear, check the baud rate and the cable; if it still does not, see the troubleshooting matrix below.
6. Once the CPU is visible, choose PLC → Upload Station to PG to copy the entire PLC project to the laptop. Save the upload as-is; do not recompile or modify anything at this point.

Reading and Interpreting the S7 Diagnostic Buffer

Open the diagnostic buffer at PLC → Diagnostic/Setting → Diagnostic Buffer (or Module Information → Diagnostic Buffer). The most recent events appear at the top. The format of each entry is:

The following table maps the entries most likely to be seen on a 16-year-old MMC100.2 lathe when alarm 810005 is active:

Save the entire diagnostic buffer to a .txt file (right-click the buffer → Save As) and attach it to your service ticket. The buffer is unambiguous evidence of root cause and protects the warranty conversation.

Fuse-Blowing: Secondary Symptom of a Short-Downstream Fault

The 5×20 mm glass fuse visible in the operator's picture is the standard 24 V DC output-group fuse on the SINUMERIK 840D MMC100.2. It is fed from the secondary of a 230 V AC → 24 V DC control transformer / regulated power supply (typically a Siemens 6EP1334-3BA00 Sitop modular 10 A power supply or its predecessor). The fuse protects the eight SM322 DO (digital-output) channels on the 24 V rail.

Three failure modes are common on a 16-year-old machine:

1. Short to frame on the 24 V DC solenoid wiring: a pinched cable against the parts-catcher carriage chassis or a hydraulic-oil-soaked cable near the spindle-lock cylinder. Measure resistance from each solenoid terminal to chassis ground with the solenoid disconnected. A reading below 1 MΩ indicates insulation breakdown.
2. Short between the 24 V DC output and the OV return on the SM322 module: the SM322 module itself has failed. This is rare but happens on MMC100.2 systems that have run hot for years. Replace the SM322 (typical order code 6ES7322-1BH01-0AA0 for 16 DO 24 V DC).
3. Defective coil / diode in the valve solenoid: the solenoid has an internal short between coil windings, drawing 1–2 A at 24 V DC and blowing the fuse within 1–2 s of energization. Disconnect each solenoid in turn (a hydraulic technician can do this in 5 min) and re-energize the circuit to identify the offending coil.

Procedure to isolate the fault:

1. Switch off the 24 V DC supply at the cabinet isolator (typically a Siemens 5SY2 / 5SP2 miniature circuit breaker / 3RV1 motor protector). Verify zero voltage with a Fluke 87V or equivalent on the load side.
2. Pull all field wiring off the SM322 DO module that feeds the parts catcher and spindle lock. Label every conductor.
3. Use a Megger MIT420 or a standard insulation tester to measure from each conductor to chassis ground. The reading should be ≥ 100 MΩ. Any reading below 1 MΩ is a short.
4. Reconnect the conductors one at a time, powering the 24 V DC and observing the fuse and the PLC error bits. The fuse will blow when the shorted branch is reconnected.
5. Once the shorted branch is found, repair the cable (heat-shrink, replace gland, route away from moving parts) and re-energize.

Re-loading the NC PLC Backup Data

If the diagnostic buffer shows a checksum error, lost retentive data, or a corrupt MMC card, the next step is to reload the NC PLC backup. Siemens strongly recommends doing this only with a known-good backup that matches the exact machine configuration.

1. From the HMI: Start-up → Commissioning → NC/PLC → NC-PLC Data Transfer. The path is menu-driven and depends on the MMC version; on MMC100.2, the sub-tree is Start-up → NC/PLC → Commissioning.
2. Locate the backup .arc file (Siemens archive format) on the CF card of the HMI (typical path: D:\DH\ARC or F:\CARD\DH\ARC.DIR\*). The customer should have at least one .arc file dated around the time of the original commissioning.
3. Choose Read in and select the matching .arc file. The system will display a series of prompts for the password (default Siemens: SUNRISE on a fresh 840D Powerline, but the customer should know their own password).
4. Once read in, the NCU will automatically perform a warm restart and reload the PLC into the S7-300. The 810005 alarm should clear if the reload was clean.
5. Verify the alarm is gone: power-cycle the machine, leave it idle for 5 minutes, and confirm the HMI alarm line is empty.

Verification and Commissioning Checks

After the PLC backup is reloaded, run the following checks before returning the machine to production. Each row is a single-pass / fail test; record the result for the service ticket.

Troubleshooting Matrix for 810005 + Blowing Fuse

The combined symptom of "alarm 810005 + PLC user alarms + blowing fuse" has a small set of possible root causes. The matrix below is the field-proven decision tree.

Spindle-Lock and Parts-Catcher Signal Chains (Reference)

The two sub-systems that raised the user alarms are the most safety-relevant auxiliary systems on a CNC lathe. The table below shows the typical I/O and timing for each.

All four sub-systems share the same 24 V DC output group and the same 6.3 A fuse. A single short in any of them can blow the fuse and prevent all four from operating. The cross-checking procedure in Fuse-Blowing: Secondary Symptom of a Short-Downstream Fault above isolates which sub-system is shorted.

Spare Parts Recommendation for a 16-Year-Old MMC100.2

For a machine in service for 16+ years, the following spares are strongly recommended and should be on the shelf before the next failure:

Reference Documentation

Refer to the following official Siemens documents while performing this diagnosis. The URLs below are the official Siemens Industry Online Support entries and should be opened with a registered Siemens MySupport account for full PDF access.

• SINUMERIK 840D sl/840Di sl/828D Operator Components Manual (08/2014, 6FC5397-1CP40-3BA0)
• SINUMERIK 840D sl/840Di sl/828D Function Manual (01/2019, 6FC5397-3AP40-3BA0)
• SINUMERIK 840D/810D Commissioning Manual (Edition 04.96, 6FC5297-1AD10-0AP0)
• SINUMERIK 840D sl/840Di sl/828D Diagnostics Manual (01/2018, 6FC5398-6AP40-3BA0)
• SIMATIC S7-300 CPU 314 Manual (6ES7390-1AE80-0AA0)
• SINUMERIK 840D sl/840Di sl/828D HMI sl Manual (10/2015, 6FC5397-1BP40-3BA0)