S7-300 CPU314 STOP Mode from SM322 SF2 Fault Diagnostics

1. Problem Overview

An S7-300 CPU314 in a Mainline Pump-A control system drops to STOP after a SM322 digital output module (MLFB 6ES7322-1FL00-0AA0) reports an SF2 group fault. The red SF2 LED is illuminated on one of the four output groups of the 32-channel DO card, the CPU STOP LED is steady, and a power-on reset does not bring the controller back to RUN. Pumps B and C, wired to other DO cards in the same rack, are no longer controllable because the user program is not executed while the CPU is stopped.

This article documents the exact diagnostic path, the fault OB strategy, and the field-proven card-swap workaround used to restore pumping while a replacement module is sourced.

2. What the SF and SF2 LEDs Mean on the SM322

The SM322 (6ES7322-1FL00-0AA0) is a 32-channel digital output module organised as four groups of eight outputs. The front-panel LED stack is:

According to the TIA Portal functional description for S7-300/400 STOP mode, when the CPU goes to STOP the user program is no longer executed and the process outputs are switched to a defined state — typically substitute value 0 (de-energised), unless configured for "Keep last value". For AC output modules this means every contactor fed by the SM322 drops out, which is the immediate cause of the pumps stopping.

3. SM322 6ES7322-1FL00-0AA0 — Module Specifications

The module referenced in the field report is fully specified as follows. These values come from the official Siemens product page on the Industry Mall and the S7-300 module data manual.

4. Why the CPU Enters STOP — Fault OB Mechanism

An S7-300 CPU does not enter STOP because an output is bad; it enters STOP because the diagnostic event has no Fault OB loaded in the user project. The mechanism is:

1. The SM322 detects the overcurrent/short and writes a diagnostic interrupt to the backplane.
2. The CPU receives the diagnostic interrupt and looks for OB82 (diagnostic interrupt) in the project.
3. If OB82 is missing, the CPU goes to STOP and logs the event in the diagnostic buffer with the text "OB82 not loaded — STOP due to diagnostic interrupt".
4. On the next power-on, the same fault is detected, the same OB82 is missing, and the CPU goes straight back to STOP — which is exactly the symptom reported after the user's power reset.

Other Fault OBs that can drop an S7-300 to STOP when missing:

For a per-channel SM322 fault, OB82 is the OB the CPU is asking for. OB86 is the secondary candidate if a hot-swap was attempted and the CPU interpreted the missing module as a rack failure.

5. Reading the STEP 7 Diagnostic Buffer

The diagnostic buffer is the authoritative source. Open the buffer in STEP 7 (Classic) or TIA Portal:

1. Go online with the CPU (PLC > Online > Accessible Nodes or via the project).
2. Open PLC > Diagnostics/Settings > Diagnostic Buffer (TIA: Online & Diagnostics > Diagnostics buffer).
3. Click Save As and export the full buffer as a text file. The buffer is a chronological ring of the last ~200 events with millisecond timestamps.
4. Locate the most recent STOP event. The message will read something like:
   STOP due to diagnostic interrupt (OB not found: OB82)
   followed by the slot and module diagnostic data identifying the SM322 and channel group.
5. Note the slot number, the module start address (e.g. 4..7 byte for 32 DO), and the exact OB referenced in the "OB not loaded" text.

6. Immediate Workaround — Add the Missing Fault OB

If the diagnostic buffer points to OB82 (or another OB), adding an empty fault OB to the project and downloading it is the cleanest path to RUN. The empty OB performs no action but suppresses the "OB not found" STOP.

1. In STEP 7 (TIA or Classic) open the S7-300 project offline.
2. Right-click CPU314 > Program blocks and add a new block: OB82 (type: "Diagnostic interrupt").
3. Leave the body empty. Save and compile.
4. Connect online, perform a stop → download → run sequence:
   PLC > Operating Mode > STOP
PLC > Download
PLC > Operating Mode > RUN
5. The CPU will now remain in RUN even though the SM322 still reports the group fault. The other 24 channels and other DO cards in the rack continue to operate normally.

Repeat for any other Fault OB referenced in the buffer (commonly OB86 on PROFIBUS-style racks, OB85 after hot-swap, OB121 / OB122 if user code touches bad I/O).

7. Card-Swap Workaround — Run Pumps B and C on a Healthy Card

If the spare DO card is needed immediately and there is no engineering station available, the card can be physically moved. The 6ES7322-1FL00-0AA0 is a slot-addressed, byte-aligned I/O module; as long as the slot the user moves it into is configured in HW Config for the same module type, the addressing is preserved.

1. Open HW Config and identify:
   • The slot and start address of the faulty DO card (slot-A108 in the source schematic).
   • The slot and start address of the "less critical" DO card the user proposes to swap with.
2. Confirm both slots in HW Config are configured with 6ES7322-1FL00-0AA0. The configuration must match — Siemens S7-300 will reject a module whose order number does not match the configured order number (compare slot with PG).
3. Stop the CPU, isolate power to both SM322 modules (open the load-voltage disconnector for each).
4. Mark and label every terminal of both modules. Latch the front connector release and pull straight out. Each front connector carries 40 screw or spring terminals; mistaking a feed with a return will energise the wrong field device.
5. Swap the front connectors (most common mistake is the connectors, not the modules themselves — moving the modules is optional).
6. Restore load voltage. Power the rack.
7. Observe the CPU: the SF2 LED is now on a different slot. Confirm the new location of the fault with HW Config (PLC > Online > Compare).
8. Re-attempt RUN. If the diagnostic buffer now references an OB other than OB82 (e.g. the swap triggered an OB85 module-mismatch event), add the missing OB as in Section 6.

8. Can the CPU Run with a Faulty DO Card Present?

Yes — with the right Fault OBs loaded, the S7-300 CPU314 continues to RUN even while a SM322 reports an active group fault. The card's good channels and all other modules continue to be scanned and controlled. Only the affected 8-channel group on the faulty card will not respond (their electronic short-circuit protection is latched off, and the channel status bit in the process image reflects this).

Operationally this means:

• Mainline Pump-A, fed from the faulty group 2 outputs of the SM322, stays off until the card is repaired or replaced.
• Pumps B and C, fed from the swapped or independent DO card, run normally once the CPU is in RUN.
• The SF LED on the CPU stays lit and the diagnostic buffer keeps appending OB82 entries. Treat that as a maintenance alarm, not a process trip.

9. Verification Checklist

1. CPU RUN LED solid green, STOP LED off.
2. No new entries in the diagnostic buffer referencing OB not found.
3. Pump B and Pump C contactors pull in when commanded; verify with field current clamp or contactor status feedback.
4. Faulty group's outputs remain in the configured substitute value (0) and are not written from the program — confirm in the Monitor/Modify view that bits in the affected byte read back 0.
5. Order replacement module 6ES7322-1FL00-0AA0 from the Siemens Industry Mall. The MLFB is still active and not phased out.
6. Plan a window to install the new card, restore the original wiring, and remove the temporary fault OBs if process-critical interlocks no longer need the fault-tolerant path.

10. Long-Term Replacement and Compatibility Notes

The 6ES7322-1FL00-0AA0 is still an active product line in the Siemens catalog and is not listed as phase-out. Lead time depends on regional stock, but the module is orderable from authorised Siemens distributors and direct from the Industry Mall.

Direct-compatible alternatives in the same family, useful as bench stock or if a higher-current variant is acceptable:

Always re-run PLC > Compare after any module swap. The CPU will reject a non-matching order number and either re-enter STOP or flag a module mismatch in HW Config.

11. Troubleshooting Matrix

12. FAQ