Diagnosing S7-400 CPU External Fault (EXTF) and I-O Access Errors (OB122) in PCS 7

In SIMATIC PCS 7, an S7-400 CPU entering External Fault (EXTF LED solid red) accompanied by diagnostic buffer entries reporting 'I/O access error reading' (Event ID 16#2942 or 16#2943 calling OB122) after adding a hardware module (such as an FM 350-1 counter or analog card to an ET 200M station) occurs when the controller program attempts to read an address outside the configured Process Image Partition (PIP) or when a Configuration in RUN (CiR) download was executed without proper slot parameterization. If organization block OB122 is present in the CPU, the controller remains in RUN while illuminating EXTF; if OB122 is missing, the CPU immediately trips to STOP mode.

S7-400 I/O Addressing Architecture: Process Image vs. Peripheral Access

In SIMATIC S7-400 and S7-410 controllers, data exchange with field instrumentation is structured across two memory tiers:

The Process Image (PII / PIQ): A dedicated memory table updated cyclically by the CPU operating system. In traditional STEP 7, the default process image is updated at the beginning and end of OB1. In PCS 7, to service high-speed closed-loop control loops executing in cyclic interrupt OBs (OB30 through OB38), the process image is divided into Process Image Partitions (PIP / TPA - Teilprozessabbild). Each PIP is assigned to a specific OB (e.g., PIP 1 assigned to OB35, 100ms).
Direct Peripheral Access (PIW, PID, PQW, PQD): Bypasses the cyclic process image buffer to query hardware ASICs directly. Peripheral access is required for function modules (FM 350-1), communication modules (CP cards), and analog modules mapped outside the process image range.

When a program instruction attempts to read or write a peripheral address that does not exist physically, is powered down, or whose assigned interface module is faulted, the CPU operating system intercepts the bus timeout and calls OB122 (I/O Access Error).

Root Causes of I/O Access Errors (OB122) and EXTF LED Activation

When an engineer adds an FM 350-1 counter module or analog card to an ET 200M station and observes solid EXTF on the CPU, technical evaluation identifies four primary causes:

Address Unassigned to Process Image Partition (PIP): In CFC, technological driver blocks (Pcs7AnIn, CNT_CTL1) execute inside cyclic interrupt OBs (e.g., OB35). If the module's I/O address in HW Config (e.g., PIW512) is assigned to the default OB1 process image or left unassigned, but the CFC chart executes in OB35, the driver attempts a peripheral access that conflicts with cyclic OS updating, triggering event 16#2942.
Missing Electronic Module or Backplane Bus Fault: The hardware configuration was downloaded to the CPU, but the physical module has not been inserted into the ET 200M active bus module (TM 195), the front connector is unseated, or auxiliary 24V DC backplane power is absent. The CPU attempts to read the slot, encounters zero response, and flags EXTF.
Incomplete Configuration in RUN (CiR) Sequence: If the module was added in RUN mode using CiR, but the physical module was inserted before the CiR download completed, the IM 153-2 interface module detects an unexpected card, flagging a configuration mismatch.
Diagnostic OB122 Role: The CPU illuminates EXTF (External Fault) to warn operators of a hardware failure. If OB122 is present in the CPU block folder, the CPU logs the fault and stays in RUN. If OB122 was deleted, the CPU crashes into STOP mode.

Process Image Partitioning and Diagnostic Interrupt Matrix

The following table details diagnostic event codes, error organization blocks, and process image partition rules:

Diagnostic Event CodeTriggering ConditionInvoked System OBCPU Behavior if OB PresentCPU Behavior if OB MissingEvent 16#2942I/O access error reading peripheral addressOB122Stays in RUN; EXTF LED ONTrips to STOP modeEvent 16#2943I/O access error writing peripheral addressOB122Stays in RUN; EXTF LED ONTrips to STOP modeEvent 16#38C4Distributed I/O station failure / returnOB86Stays in RUN; EXTF/BUSF LEDTrips to STOP modeEvent 16#3882Module diagnostic interrupt (wire break)OB82Stays in RUN; EXTF LED ONTrips to STOP modePIP 1 to PIP 8Process Image Partition linked to OB3xOS updatingDeterministic cyclic I/O updateDirect PIW access required

Step-by-Step Procedure to Resolve I/O Access Errors and Clear EXTF

To eliminate I/O access errors and clear the EXTF fault LED on the S7-400 CPU:

Analyze the Diagnostic Buffer in SIMATIC Manager:
- Select the CPU in SIMATIC Manager; open PLC -> Diagnostic/Setting -> Module Information.
- Under the Diagnostic Buffer tab, locate the event: 'I/O access error reading'. Note the exact peripheral address (e.g., Address: 512) and block caller (e.g., FC... or FB...).
- Note whether event ID is 16#2942.
Check Process Image Partition (PIP) in HW Config:
- Open HW Config for the S7-400 station.
- Double-click the newly added module (e.g., FM 350-1 or analog card).
- Open the Addresses tab.
- Inspect the Process image dropdown. If the module is read by a CFC chart running in OB35 (100ms), assign the process image to PIP 1 (or whichever PIP corresponds to OB35 in CPU properties).
- If the module must be accessed directly via peripheral addresses (e.g., FM 350-1 with CNT_CTL1), select None (direct peripheral access) and verify that the base address does not overlap existing cards.
Inspect Physical Module Seating and Power:
- Check the physical ET 200M rack: Confirm that the module's front panel SF (red) LED is not solid, and that the 24V DC auxiliary power connector is wired securely.
- Verify that the module is firmly seated into its active bus unit (6ES7195-7HB00-0XA0).
Ensure Error Organization Blocks Are Present:
- In the CPU offline Blocks folder, confirm that OB122, OB82, OB83, OB85, and OB86 exist.
- If missing, copy them from the standard library and download to the CPU to guarantee STOP prevention.
Compile and Download Hardware and CFC:
- In HW Config, execute Station -> Save and Compile.
- Download the hardware configuration to the S7-400 CPU.
- In CFC, execute Chart -> Compile -> Changes Only with module driver generation enabled, and download changes.

Verification

Verify fault clearance and normal operation:
1. Inspect the S7-400 CPU front panel: Confirm that the EXTF LED turns completely OFF.
2. Inspect the newly added module: Confirm that the module SF LED turns completely OFF.
3. In the CPU diagnostic buffer, verify that zero new entries for Event 16#2942 appear.
4. Open the technological CFC chart online; verify that the driver block reads valid field signals with Quality Code 16#80.

Limitations and Operational Precautions

Never Mask Faults Indefinitely with OB122: While OB122 prevents the CPU from entering STOP mode, running with continuous I/O access errors consumes CPU cycle time and blinds operators to genuine field failures.
CiR Memory Reservation: When using Configuration in RUN (CiR), never exceed the pre-allocated slave or module limit defined in the original hardware compilation.
Process Image Size Ceiling: S7-400 CPUs have configurable maximum process image boundaries (e.g., up to 16 KB). Verify in CPU Memory properties that address assignments do not exceed configured limits.

Engineer FAQ

What does solid EXTF LED mean on an S7-400 CPU?

EXTF (External Fault) indicates a hardware failure outside the CPU, such as an I/O access error (OB122), a missing distributed I/O rack (OB86), or a field wire break (OB82).

Why didn't the CPU enter STOP mode during the I/O access error?

Because organization block OB122 (I/O access error OB) is present in the CPU. If OB122 is loaded, the CPU handles the exception in RUN mode.

What is a Process Image Partition (PIP) in PCS 7?

A PIP is a subsection of the process image updated cyclically with a specific cyclic interrupt OB (such as OB35), ensuring synchronized data exchange for fast control loops.