1. Problem Summary
The SIMATIC ET 200S IM151-8 PN/DP interface module with integrated CPU rejects a STEP 7 V5.5 SP2 download with the following combined fault picture:
- Hardware configuration downloads successfully into the target.
- System data (SDB container) is refused with Download (13:4718) Cannot load SDB 7 and Download (13:4589) The system data cannot be loaded to module IM151-8 PN/DP CPU - (R0/S2).
- Any subsequent online operation, including opening OB1, fails with Online: Communication cannot be established. The connection partner is not responding.
- The PG/PC interface selector is locked, so the user cannot pivot to an alternate adapter while the CPU remains in this state.
This fault is one of the most common first-commissioning failures on the IM151-8 family. It appears almost immediately after the user believes Ethernet connectivity is healthy, which makes the symptom confusing because PG-to-CPU pings and even partial HWConfig transfers can succeed before the download is aborted.
2. Affected Hardware and Software Stack
| Item | Value in the field case | Notes |
|---|---|---|
| CPU / IM | 6ES7151-8AB01-0AB0 (IM151-8 PN/DP, 3-port switch) | Firmware V3.x and V4.x affected identically |
| Engineering tool | SIMATIC Manager V5.5 + SP2 + HF7 (or later) | Same root cause applies to TIA Portal V13 SP1 and newer |
| PG/PC interface | TCPIP.1 — Realtek PCIe GBE Family Controller | Any RFC 1006-capable NDIS adapter is acceptable |
| PLC IP | 192.168.1.5 / 24 | Default PROFINET device name not yet assigned |
| PG IP | 192.168.1.1 / 24 | Same /24, no router required |
| Load memory | None (no MMC inserted) | Root cause |
3. Error Code Reference
STEP 7 surfaces this failure as a stack of three diagnostic events. Each line has independent meaning and is documented in the STEP 7 online help. The catalog values below are taken from the SIMATIC diagnostic event reference shipped with STEP 7 V5.5 SP2.
| Event ID | Plain-text message | What it actually means |
|---|---|---|
| 33:279 | The block / operating system update is too large | The SDB container (including SDB 0, SDB 7, SDB 1000+) exceeds the available load memory. The IM151-8 has no integrated load memory; it relies entirely on the plug-in SIMATIC Micro Memory Card (MMC). |
| 13:4718 | Cannot load SDB 7 | SDB 7 is the system-side SDB that defines the IO configuration, PROFINET device substitution, and basic CPU parameters. It is the first SDB written; failure here blocks all subsequent SDBs. |
| 13:4589 | System data cannot be loaded to module IM151-8 PN/DP CPU | Wrapper event raised by the CPU's system loader pointing at the diagnostic buffer of the named slot (R0/S2 = Rack 0, Slot 2). |
The SDB family in question is documented in the STEP 7 online help under System Data Blocks (SDB). SDB 7 in particular is generated by HWConfig and is mandatory for any ET 200S station; it is not an optional system block.
4. Root Cause Analysis
The IM151-8 PN/DP CPU does not contain non-volatile load memory on board. Like the S7-300 CPUs (CPU 312, 314, 315-2 PN/DP, 317, 319), the IM151-8 requires a SIMATIC Micro Memory Card (MMC) for:
- Storing the user program (OB, FB, FC, DB, SDB, SFC, SFB).
- Storing the generated SDB container from HWConfig.
- Storing firmware updates in update mode.
- Archiving project data and recipes.
Without an MMC, the integrated CPU boots into a degraded state in which the PN/DP interface initializes, accepts the TCP/ARP handshake, and even permits a partial HWConfig transfer. It then refuses the system data write because there is no non-volatile destination.
The subsequent connection partner is not responding message is not a network fault — it is a side-effect of the CPU tearing down its S7 connection table when the SDB commit fails. The PG/PC interface selector locks because the active S7 connection state machine never completed handshake.
5. Required Memory Card Specification
Use only genuine Siemens SIMATIC Micro Memory Cards. Third-party SD-form-factor cards in MMC adapters are not supported and will not be recognized by the IM151-8 bootloader.
| MLFB | Size | Typical use | Notes |
|---|---|---|---|
| 6ES7953-8LF20-0AA0 | 64 KB | Small programs, no recipe archives | Minimum supported on IM151-8 |
| 6ES7953-8LG20-0AA0 | 128 KB | Typical ET 200S project | Recommended default |
| 6ES7953-8LJ20-0AA0 | 256 KB | Mid-size projects with C-bus modules | Safe margin for SDB growth |
| 6ES7953-8LL20-0AA0 | 512 KB | Large projects, F-modules, recipes | Recommended for F-CPU variants |
| 6ES7953-8LM20-0AA0 | 1 MB | Extensive libraries or trend archives | Largest MMC for S7-300 / IM151-8 |
| 6ES7953-8LP20-0AA0 | 2 MB | Reserved for future expansion | Verify firmware support |
For an IM151-8 with a 16-byte I/O configuration, a 128 KB MMC (6ES7953-8LG20-0AA0) is the practical minimum and provides comfortable headroom for future SDB growth as modules are added. F-CPU variants of the IM151-8 require at least 256 KB.
6. Step-by-Step Resolution
6.1 Prerequisites
- Powered-down IM151-8 station (24 V DC off).
- Suitable MMC from the table above.
- SIMATIC Manager V5.5 SP2 with matching HWConfig catalog (GSD files installed for any non-Siemens head modules).
- Ethernet patch cable (Cat 5e or better) with 192.168.1.5 / 192.168.1.1 endpoints already verified by PING.
6.2 Procedure
- Power down the station. Remove the 24 V DC connector or open the PS coupler breaker.
- Insert the MMC. The slot is on the front of the IM151-8 under the upper door. The card is keyed; orientation is enforced. Push until it latches flush. The CPU will not detect a card inserted while powered — it must see the card at boot.
- Power up the station. Wait for the SF, BF, and MT LEDs to complete their startup sequence. The CPU should reach STOP with SF off and DC5V on, indicating a healthy boot.
- Re-open SIMATIC Manager on the PG and confirm the PG/PC interface selector is now unlocked. If still locked, close and re-open the STEP 7 project.
- Select the correct TCPIP.1 access point. Options → Set PG/PC Interface → TCPIP.1 — Realtek PCIe GBE Family Controller. Do not choose the Realtek auto-detect entry; the explicit node assignment is required.
- Open the S7 project, then PLC → Download from HWConfig. Allow the entire container (SDB 0, SDB 7, SDB 1000+) to commit. The download should now complete with no 13:4718 event.
- Download blocks (OB1, FB, FC, DB). Confirm that OB1 opens in online view without the connection-partner error.
- Place the CPU in RUN via PLC → Operating Mode or the toggle on the front of the IM.
7. Network and PG/PC Interface Verification
Even after the MMC fix, intermittent "connection partner is not responding" events are usually network-related, not CPU-related. Verify the following before chasing another firmware bug:
| Check | Expected result | Failure action |
|---|---|---|
ping 192.168.1.5 -t from PG |
<1 ms reply, no loss | Re-terminate cable, disable Windows firewall |
arp -a on PG |
MAC of IM151-8 listed as dynamic | Static ARP entry not required but helps on some NICs |
| PG/PC interface assignment | TCPIP.1 bound to the physical NIC only | Disable Hyper-V, VirtualBox, VMware virtual NICs |
| Subnet match | Both nodes in 192.168.1.0/24 | Correct mask, no off-subnet gateway |
| PG socket ports | ISO-on-TCP port 102 open, not blocked | Add firewall rule for TCP/102 and UDP/34964 |
| PROFINET device name | Empty or matches project | Assign via PLC → Ethernet → Assign Device Name |
8. Diagnostic Buffer Analysis
After the MMC is inserted and the station re-booted, drain the diagnostic buffer to confirm there are no residual errors:
- In SIMATIC Manager, PLC → Diagnostic/Setting → Diagnostic Buffer.
- Sort by Time descending. The most recent event should be CPU restart (warm) with no SDB error following it.
- If SDB events are still present, the MMC is either too small or write-protected. Check the lock slider on the MMC body.
The diagnostic buffer is the authoritative record of what the CPU has actually accepted. A clean buffer immediately after a power-cycle is the proof of a successful recovery.
9. Hardware Configuration Re-Download (HWConfig)
If the original SDB commit left the CPU in an inconsistent state, force a clean rewrite of the station:
- Open HWConfig for the SIMATIC 300(1) station.
- Station → Save and Compile (Ctrl+F7). This regenerates the SDB container including SDB 7.
- PLC → Download to Target System. In the Select Target Modules dialog, enable Replace object data.
- Answer Yes to the Do you want to load the system data with the project data? prompt.
- Watch the Download progress window for any residual 13:4xxx or 33:279 events. A successful run ends with Download (0:0) successfully completed.
The "replace object data" option is critical on the IM151-8 because partial SDB commits from a failed first attempt will persist otherwise and the next download will be a no-op.
10. Verification and Functional Check
- Online connection test: PLC → Accessible Nodes. The IM151-8 must appear with its IP and PROFINET device name.
- OB1 online open: Right-click OB1 → Open / Monitor. No connection error should appear.
- Force table: Create a VAT, enter a tag, monitor online. Forces should apply if the test program writes them.
- PROFINET topology check: In HWConfig, PLC → PROFINET IO → Topology. All ports must show link > 100 Mbps / full duplex.
- Diagnostic buffer clean: No events newer than the most recent warm restart.
- Power-cycle test: Pull 24 V for 10 s, restore, confirm project reloads from MMC without PG intervention.
11. Preventive Measures and Best Practices
- Always fit an MMC before the first download. Add the MMC presence check to the commissioning checklist.
- Label the MMC with the project name, version, and date. A mis-swapped MMC is the most common cause of a working station suddenly returning to the 13:4718 fault after a maintenance visit.
- Keep a known-good spare MMC in the cabinet. Field recovery time for a missing MMC is dominated by the procurement delay, not the diagnostic effort.
- Avoid SD-to-MMC adapters. They are not recognized by the IM151-8 bootloader and will produce the same SDB 7 refusal as no card at all.
- Reserve 30% free MMC capacity. SDB growth as modules are added is silent; an MMC that is 95% full today will fail the next expansion.
- Document the MMC capacity in the project. Add the capacity to the project header and to the station label so future engineers do not have to repeat this diagnosis.
12. Migrating to TIA Portal
The same root cause — missing MMC — applies when the project is later migrated to TIA Portal V13 SP1 or newer. The error wording differs but the diagnostic buffer entries are identical:
| STEP 7 V5.5 event | TIA Portal equivalent | Meaning |
|---|---|---|
| 33:279 | "Block / system update too large" | Load memory exhausted |
| 13:4718 | "Cannot download system data (SDB 7)" | SDB commit refused |
| 13:4589 | "System data cannot be loaded to IM151-8 PN/DP" | Wrapper event |
TIA Portal does not surface the "PG/PC Interface locked" symptom because the interface selector is replaced by the project tree's Online → Accessible Devices view. After inserting the MMC and power-cycling, the IM151-8 will appear in the accessible nodes list immediately.
What does the "Download (13:4718) Cannot load SDB 7" error mean on the IM151-8 PN/DP?
It means the CPU refused to commit SDB 7 — the system data block generated by HWConfig that defines the station's IO and basic parameters. On the IM151-8 PN/DP this happens because the CPU has no integrated non-volatile load memory and the project is being downloaded without an MMC installed.
Do I really need a memory card for the ET 200S IM151-8 PN/DP CPU?
Yes. The IM151-8 has no internal load memory; it requires a genuine SIMATIC Micro Memory Card (MLFB 6ES7953-8Lxx0-0AA0) for every download. A 128 KB MMC (6ES7953-8LG20-0AA0) is the recommended minimum for a non-F station.
The PING to 192.168.1.5 succeeds but STEP 7 still says "connection partner is not responding" — why?
The PN/DP interface is up but the integrated CPU has torn down its S7 connection table because the SDB commit failed. Re-establishing TCP/IP does not rebuild the S7 connection; you must fix the load-memory problem (insert MMC), power-cycle, and re-download HWConfig.
Why is the PG/PC interface selector greyed out in SIMATIC Manager?
SIMATIC Manager locks the PG/PC interface while a half-open S7 connection exists. After fixing the SDB problem and power-cycling the CPU, close and re-open the STEP 7 project to unlock it.
Can I use a third-party SD card in an MMC adapter instead of a Siemens MMC?
No. The IM151-8 bootloader does not recognize SD-to-MMC adapters. Use only genuine Siemens SIMATIC Micro Memory Cards. A non-genuine card will produce the same SDB 7 refusal as no card at all.
Will the same fix work if I migrate the project to TIA Portal?
Yes. The root cause is hardware, not software. Insert the MMC, power-cycle the IM151-8, and the device will appear in TIA Portal's Online → Accessible Devices view. The TIA Portal error wording differs but the diagnostic buffer events 33:279, 13:4718, and 13:4589 are identical to STEP 7 V5.5.