Resolving IM151-8 PN/DP Error 33:279 System Data Load Failure

Problem Statement

When downloading a STEP 7 hardware configuration to a Siemens IM151-8 PN/DP CPU from SIMATIC Manager, the following cascade of errors blocks the transfer:

1. Download (33:279): The block/op. sys. update is too large.
2. Download (13:4718): Cannot load SDB 7.
3. Download (13:4589): The system data cannot be loaded to module IM151-8 PN/DP CPU - (R0/S2) of station SIMATIC 300(2).

The IM151-8 PN/DP signals the failure in its diagnostic buffer with the same text. Slot indication R0/S2 means the head module (the IM itself) at Rack 0 / Slot 2 of the ET 200S station rejects the system data blocks (SDBs) that STEP 7 attempts to push across PROFINET or PROFIBUS DP. The CPU never returns a SF (group fault) caused by silicon failure — every documented case traces to the load memory medium: an undersized MMC, a worn MMC, or the absence of an MMC altogether. The ET 200S IM151-8 PN/DP family does not contain integrated load memory, so the MMC is mandatory for both program and configuration persistence.

Affected Hardware and Order Numbers

The error pattern is specific to the ET 200S distributed I/O with integrated CPU line. Verify the MLFB on the front panel sticker before applying this procedure.

All variants use the same 3.3 V SIMATIC Micro Memory Card (MMC) and accept the same error codes. A failed download on a 6ES7151-8AB01-0AB0 behaves identically to one on a 6ES7151-8FB01-0AB0 because the error originates from the load-memory manager, not the work-memory manager.

Error Code Decoding

STEP 7 raises three related entries because the download pipeline reports the failure at three layers. Treat them as one fault and address the MMC.

Reading the diagnostic buffer is mandatory. Open SIMATIC Manager » PLC » Diagnostic/Setting » Diagnostic Buffer, sort by time, and confirm the System data cannot be loaded to module IM151-8 PN/DP CPU entry. The buffer frequently shows a second event immediately above it — MMC error or Flash write error — which is the smoking gun for MMC wear.

Root Cause Analysis

Four root causes are documented for the 33:279 / 13:4589 combination on IM151-8 PN/DP CPUs. Confirm each before ordering parts.

1. Missing MMC. The CPU boots but has no load memory. STEP 7 sees zero free bytes and rejects the project.
2. Undersized MMC. The project (blocks + SDBs + comments) needs more bytes than the card offers. Typical with 64 KB or 128 KB MMCs on stations with many SDBs.
3. Worn or aged MMC. SIMATIC MMCs use NAND flash with a finite erase-cycle budget. Old cards (10+ years, or cards that have been overwritten thousands of times) develop bad blocks. STEP 7 reports success on the first write attempt and then surfaces flash write error on the next SDB commit. The visible symptom is identical to an undersized card.
4. Read-only or write-protected MMC. The write-protect slider on the MMC carrier is in the lock position. The IM151-8 PN/DP respects the slider and refuses any write.

Field experience shows that root cause #3 (wear) is the most common in working plants where the same MMC has been re-flashed dozens of times during commissioning or by repeated backup operations. The fix is a new MMC of equal or larger capacity.

LED and Diagnostic Indicators

Before touching the card, observe the front-panel LEDs. They narrow down the failure mode.

If SF and STOP are both steady and RUN is dark, the CPU is recoverable. Proceed to the solution.

Solution: MMC Replacement and Re-Download

The corrective procedure is a controlled MMC swap. The PLC must be in STOP and the supply voltage must remain on so the diagnostic buffer survives. Never hot-plug the MMC on a running CPU — the IM151-8 PN/DP does not support MMC hot-swap and may flag a memory-reset event.

Prerequisites

• Replacement SIMATIC MMC sized for the project (4 MB minimum, 8 MB recommended).
• STEP 7 V5.5 + SPx or compatible on the PG/PC.
• Online connection to the IM151-8 PN/DP via PROFINET or PROFIBUS DP.
• The project archive (backup) on the PG/PC hard disk.

Approved MMC Order Numbers

Step-by-Step Procedure

1. Place the CPU in STOP. Set the mode switch to STOP, or send PLC » Diagnostic/Setting » Operating Mode » STOP from SIMATIC Manager.
2. Open the diagnostic buffer. Confirm the 13:4589 entry and note any Flash write error events above it. This validates the MMC-wear hypothesis.
3. Power down the station. De-energize the ET 200S head and I/O modules. The IM151-8 PN/DP does not retain the diagnostic buffer through a power cycle, so capture the buffer contents to a text file first (Save as... in the diagnostic buffer dialog).
4. Remove the old MMC. Press the ejector tab, slide the card out. Inspect the gold contacts for oxidation or contamination. A worn card often shows a dull, tarnished contact surface even when the plastic looks fine.
5. Insert the new MMC. Slide the card in with the label facing forward until the ejector clicks. Verify the write-protect slider is in the unlocked position (the small tab away from the gold edge).
6. Power up the station. The CPU performs a memory reset. The STOP LED flashes at ~2 Hz during reset, then stays steady. SF should extinguish if the MMC is healthy and no other faults exist.
7. Re-establish the online connection. In SIMATIC Manager, PLC » Download to Target System (or Ctrl+L). STEP 7 will offer to format the new MMC — accept. Formatting takes 10–30 seconds for a 4 MB card.
8. Download the project. Select Download to Target System » Hardware and Software (entire project). The HW Config transfer writes SDBs first, then the OB/FB/FC/DB blocks, then the comments. Watch the "Download to target system" progress dialog for any red entries.
9. Place the CPU in RUN. Switch the mode selector to RUN, or use PLC » Operating Mode » RUN. The RUN LED should come on within 2 seconds of OB1 startup.
10. Re-check the diagnostic buffer. Confirm no new fault entries appeared during the download.

Verification Checklist

Memory Sizing Calculation

Use this rule of thumb to pick the right MMC capacity and avoid repeat failures:

1. In SIMATIC Manager, mark the project and choose Edit » Object Properties » Blocks and note the total block size (code + data + comments).
2. Multiply the SDB footprint by 1.5: every SDB occupies flash, and STEP 7 reserves a contiguous free pool equal to the largest single SDB.
3. Add firmware size (~1.5 MB for the IM151-8 PN/DP if a firmware update is staged on the card).
4. Round up to the next catalog MMC size.

For a 6ES7151-8AB01-0AB0 with ~80 KB of blocks and ~40 KB of SDBs (small PROFINET station), a 4 MB MMC gives 5× headroom and is the safest minimum.

Edge Cases and Field-Proven Caveats

These conditions produce the same 33:279 symptom but need a different fix.

• PG/PC routing mismatch. When the IM151-8 PN/DP sits behind a PROFINET switch and the routing table in HW Config is stale, STEP 7 opens an online session, then fails on SDB 0. The fix is PLC » Ethernet » Edit Ethernet Node » assign the device a fresh IP and re-download.
• Firmware-version mismatch. If the SDB references a module that requires a newer IM151-8 PN/DP firmware, the CPU accepts the SDB and then refuses to enter RUN. The download itself succeeds; the 33:279 here is a *secondary* symptom. Update the CPU firmware with a separate MMC-based FW update and retry.
• Write-protect slider engaged. The CPU does not report a clear "card is read-only" error. It reports 33:279 on the first block and 13:4589 on the first SDB. Always check the slider.
• PG/PC firewall blocking ISO-on-TCP. The download to an IM151-8 PN/DP via PROFINET uses TCP/UDP and is not affected. If the project is downloaded via the PROFIBUS DP port with PC Adapter USB, Windows firewall rules can drop the ISO protocol silently. The transfer appears to start, then halts at SDB 7 with 13:4718.
• Time on the CPU is wrong. After a successful MMC swap, the diagnostic buffer timestamps will be 2002 or earlier because the RTC lost its power backup. Set the time via PLC » Diagnostic/Setting » Set Time of Day; this prevents misleading buffer entries later.

Preventive Maintenance

To keep the IM151-8 PN/DP healthy and the next download uneventful:

• Cycle MMCs every 3–5 years in high-write installations, regardless of LED state. Flash degradation is silent until a write hits a bad block.
• Avoid using the same MMC as both a transport medium and a permanent load memory. Carry a separate MMC per controller in the cabinet and label them with the date of first insertion.
• After every successful download, export the diagnostic buffer to a .txt file and archive it with the project backup. A 1 KB buffer dump is the cleanest evidence of what was on the card before a failure.
• Keep the firmware on the CPU and the STEP 7 version in sync. A STEP 7 SP that is several versions ahead of the CPU's FW can produce SDBs the CPU cannot parse, and the 33:279 message will be misleading.

Troubleshooting Matrix