S7-300 CPU 314 Event ID 16#4550 DEFECTIVE: Diagnosis and MRES Recovery

Affected hardware: SIMATIC S7-300, CPU 314, order number 6ES7 314-1AG14-0AB0
Symptom: CPU transitions from RUN to DEFECTIVE operating mode; diagnostic buffer records Event ID 16#4550 with the message "DEFECTIVE: internal system error".
Resolution path: Power-cycle, perform a master reset (MRES), reload the STEP 7 project; replace the CPU only if the fault recurs after the reset.

1. Problem Description

A SIMATIC S7-300 station built around a CPU 314 (MLFB 6ES7 314-1AG14-0AB0) drops out of RUN and the front-panel LEDs indicate a fault. When the engineer opens the diagnostic buffer from STEP 7 (or the TIA Portal) the most recent entry reads:

Event 10 of 10:  Event ID 16#4550
DEFECTIVE: internal system error
  Z1   = 0xF013   (no relevance for user)
  Z2   = 0x0460   (no relevance for user)
  Z3   = 0x505B   (no relevance for user)
Previous operating mode : RUN
Requested operating mode: DEFECTIVE
Incoming event
Time stamp               : 02:35:03.116  07/12/1998
Coding                   : 16#4550 FF8D C450 F013 0460 505B

The buffer entry is a "stop-code event": the CPU is signalling that an internal self-test, watchdog, or firmware consistency check failed. The four context words FF8D, C450, F013, and 0460 are Siemens-internal diagnostics — Siemens documents them as "no relevance for user" because they require internal firmware logs to decode.

2. Where the Diagnostic Buffer Lives

Every S7-300 CPU and most signal/function modules keep their own diagnostic buffer — a non-volatile ring buffer that records the last ~100 diagnostic events with millisecond time stamps. Per the official Siemens TIA Portal diagnostic buffer documentation:

The CPU diagnostic buffer records all diagnostic events of the CPU and of modules connected via the backplane bus in the order in which they occurred. Each entry contains the event ID, the time stamp, and additional information (operating mode transitions, Z1–Z3 context words, etc.).

Open the buffer in STEP 7 V5.x:

1. Establish an online connection to the CPU (MPI/PROFIBUS/PROFINET, the same subnet the CPU is configured for).
2. Right-click the CPU in SIMATIC Manager > S7 Program > Blocks or in Accessible Nodes.
3. Select PLC > Module Information (German: Zielsystem > Baugruppenzustand).
4. Switch to the Diagnostic Buffer tab (German: Diagnosepuffer).

Open the buffer in the TIA Portal (V13 and later):

1. Project tree → Devices & networks → select the CPU.
2. Online > Online & diagnostics → Diagnostics → Diagnostic buffer.
3. Click any entry to see expanded information on the right pane: event ID, Z1–Z3, mode transition, and a Siemens-supplied plain-language text when one exists.

3. What Event ID 16#4550 Means

The S7-300 firmware maps diagnostic event IDs into classes by the high-order bits of the ID. The class 0x45xx covers internal firmware-detected faults. Siemens publishes the plain-language text for the class, not the sub-codes:

Event ID 16#4550 is therefore in the "DEFECTIVE — internal system error" family. The sub-code FF8D together with the Z1/Z2/Z3 words is firmware-version-specific. The PLC 314-1AG14-0AB0 ships with firmware V3.3; only Siemens support can decode the exact internal failure point from those four words.

4. The DEFECTIVE Operating Mode

The S7-300 has four operating modes selectable from the front-panel key switch and one internal mode set by the firmware itself:

The DEFECTIVE state is intentionally not user-selectable: the firmware uses it to mean "I have detected a condition I am not able to recover from automatically — do not trust the process image, do not run the user program, do not accept a START command". When the CPU is in DEFECTIVE:

• The SF (group fault) LED is on solid red.
• The STOP LED is on solid yellow.
• The RUN LED is off.
• A PG/PC can still connect via MPI/PROFIBUS/PROFINET and read the diagnostic buffer, but cannot perform a startup.
• Power-down and power-up does not by itself clear the DEFECTIVE state on a 314-1AG14 — you must perform MRES, otherwise the CPU will re-enter DEFECTIVE within a few hundred milliseconds of POST.

5. Root-Cause Analysis

For the 0x4550 family, the practical causes fall into four buckets. Always work through them in this order because the most common one is also the cheapest to test.

5.1 Transient firmware inconsistency (most common)

Watchdog timing on a heavily loaded OB1 cycle, a corrupted bit in a download that was interrupted by a power loss, or a microSD/MMC write that was interrupted. Symptom: CPU comes back cleanly after MRES and stays in RUN. This is the case observed in the source incident.

5.2 Memory module corruption

If a Flash memory card (MMC) is fitted and was written repeatedly past its endurance, blocks can fail to re-verify on read. The CPU will throw a 0x49xx series first, but on subsequent reboots you can also see 0x4550. Test by removing the MMC, performing MRES, and observing whether the CPU holds RUN without the card.

5.3 Hardware defect on the CPU itself

Aging electrolytics on the 314-1AG14 power supply section, a failed backup battery that allowed RAM corruption of the load memory image, or an internal watchdog failure. Symptom: MRES does not clear the DEFECTIVE state, or the CPU re-enters DEFECTIVE within minutes/hours of going to RUN. Requires CPU replacement.

5.4 Backplane / rack fault

Less common for 0x4550 specifically, but a loose IM360/IM361 interface module or a backplane connector with oxidized pins can cause the CPU to flag an internal error. Reseat the CPU in its slot, clean the backplane connector with isopropyl alcohol, and re-test.

6. Pre-Reset: Preserve the Evidence

Before touching the key switch, capture everything you can read while online:

1. Save the diagnostic buffer — Save As... to a .txt or .csv on the engineering station.
2. Save the module information — the Diagnostics, Memory, Performance Data, and Communication tabs all carry useful state.
3. Read out the user program — PLC > Upload Station to PG (STEP 7) or Online > Upload from device (software) in TIA Portal. This pulls blocks, HW Config, and the symbolic table. If the project on the PG is stale, the upload is the only way to recover the running configuration.
4. Note the CPU operating time counter and clock time — both are written into the buffer and help correlate with field incidents.

7. MRES Procedure for the S7-300 / CPU 314

The MRES sequence on the S7-300 is a three-step key-switch operation. Do not improvise; the timing between steps is part of the protocol.

The source incident applied a power-off / power-on / MRES sequence. Both work; power-off is only necessary if the CPU will not accept the MRES sequence because it is locked in DEFECTIVE.

8. Reloading the Project

After MRES the CPU has no project. Reload the project from the PG/PC:

1. In STEP 7: select the CPU in SIMATIC Manager, then PLC > Download. Confirm the target module is the right CPU and the slot is correct.
2. In TIA Portal: project tree → CPU → Online > Download to device. TIA will check the device name, IP, and project consistency before transferring.
3. Wait for the download to complete. The CPU will normally perform a restart and transition STOP → RUN automatically if the key switch is in RUN or RUN-P.
4. If the CPU stays in STOP after download, check the diagnostic buffer for a new stop code (e.g. 0x4380 "OB not loaded" or 0x4250 "STOP due to programming error").

9. Verification

After the CPU is back in RUN:

1. LED check. RUN = solid green, SF = off, STOP = off, BF (bus fault, if any) = off. Any solid SF LED means a diagnostic event is still active — re-read the buffer.
2. Buffer check. The buffer should show the original 16#4550 entry, then a sequence of: STOP → MRES performed → restart (OB100) → RUN. If a new 0x45xx entry appears, the underlying fault is not yet cleared.
3. I/O check. Force-check critical outputs via Monitor/Modify in PG/PC online mode, then remove the force. Verify the process image matches.
4. Communication check. From the PG, open Accessible Nodes (STEP 7) or Online & diagnostics (TIA) and confirm the CPU answers the diagnostic request without timeouts.
5. Long-run monitoring. Leave the CPU in RUN for at least 24 hours of normal operation. For older 314-1AG14 hardware this is the de-facto burn-in: a transient firmware inconsistency is much more likely to manifest again within 24 hours than a hard hardware failure is to manifest within the first hour.

10. When MRES Does Not Fix It

If the CPU re-enters DEFECTIVE immediately after the third MRES step, or within minutes of going to RUN, the fault is almost certainly hardware. Escalation order:

1. Re-seat the CPU in the rack. Locking screws fully down, backplane connector clean.
2. Replace the backplane bus connector if oxidized.
3. Move the CPU to a different slot and re-test. A slot-specific fault (e.g. bent pin on the backplane) is rare but real.
4. Try a known-good power supply. S7-300 PS 307 units with marginal 24 VDC output can cause internal CPU errors on a 314-1AG14 that look like firmware faults.
5. Replace the CPU. Order the exact successor MLFB if 6ES7 314-1AG14-0AB0 is no longer in production: 6ES7 314-1AG14-0AB0 is the canonical order number; cross-check with your Siemens distributor. The 6ES7 314-1AG14 family shares the same load memory architecture and is firmware-compatible across the V3.x line, so a like-for-like swap is plug-and-play.

11. Related Event IDs to Recognise

When you read the buffer, expect to see a sequence of events leading up to and following the 0x4550. Recognise these siblings:

A buffer that shows alternating 0x4540 and 0x4550 events is a strong signal of an aging memory device. A buffer that shows 0x4550 immediately followed by a clean STOP → MRES → RUN sequence is the textbook transient case.

12. Long-Term Preventive Measures

• UPS the 24 VDC supply to the PS 307 module. A brownout during an MMC write is the most common trigger of transient 0x45xx events on the 314-1AG14.
• Replace the backup battery on the CPU every 3 years regardless of voltage check. A low or absent battery allows RAM corruption of the load memory image during power-down.
• Keep firmware current within the V3.x line. Siemens issued field updates that addressed specific 0x45xx sub-codes; check Siemens Industry Online Support for the latest firmware for 6ES7 314-1AG14-0AB0.
• Avoid MPI download of large blocks. Use PROFINET or PROFIBUS-DP for downloads over ~500 KB — the MPI link on the 314-1AG14 is comparatively slow and a download timeout can leave the load memory in an inconsistent state.
• Use an MMC rather than relying on internal Flash for retention. An MMC isolates the user program from internal Flash wear and gives a clean replacement path if a card fails.

13. Diagnostic Buffer Quick-Reference (STEP 7 / TIA)

14. One-Page Summary for the Field Engineer

1. Capture the diagnostic buffer to a file on the PG/PC.
2. Note Event ID 16#4550 = internal system error, CPU in DEFECTIVE mode.
3. Key switch → STOP. Hold at MRES 3 s, return to MRES within 1 s, return to MRES within 1 s again and hold until STOP is solid.
4. Download the STEP 7 / TIA project to the CPU.
5. Verify RUN LED on, SF LED off, buffer free of new 0x45xx entries.
6. Monitor 24 hours.
7. If the fault recurs: re-seat the CPU, swap the PS 307, then replace the CPU.

FAQ