S7-1200 OB40 Not Triggering on PROFINET Disconnect: Use OB82

Symptom: An OB40 (hardware interrupt OB) is created in TIA Portal V14 for an S7-1200 CPU 1212C DC/DC/DC (6ES7 212-1BD30-0XB0) to detect the unplugging of a PROFINET IO device, but the OB body never executes when the device is removed from the network. The MOVE instruction placed inside the OB remains inactive and no program scan ever reaches the OB. The fault persists even though the project is downloaded without errors and the PROFINET device is configured and online.

1. Affected Hardware, Firmware and Software

Reference: S7-1200 Programmable Controller System Manual (Siemens Support, 109772940).

2. Why OB40 Is Incorrect for PROFINET Diagnostics

The S7-1200 OB40 is the hardware interrupt OB. Per the TIA Portal documentation for hardware interrupt OBs 40–47, hardware interrupts are generated by:

• Rising-edge events on local digital inputs (DI).
• Falling-edge events on local digital inputs.
• High-Speed Counter (HSC) threshold-value CV = RV events.
• Comparators on the onboard analog inputs (CPU 1214C / 1215C / 1217C only).
• PTO (Pulse Train Output) finished events on FW V4.1+.

A PROFINET device disappearing from the network is none of these. It is an I/O subsystem-level fault (an IO device going to station failure or diagnosis from station). For S7-1200 / S7-1500 this triggers a diagnostic interrupt, dispatched through OB82 on the CPU. On S7-300/400 the same class of fault would also reach OB86 (rack / IO-device failure OB); however, S7-1200 only supports OB82 and does not provide OB86.

3. Root Cause Analysis

When the field engineer unplugs a PROFINET IO device, the IO controller observes the link-down event and raises a PROFINET alarm. The CPU converts that alarm into an OB call based on the alarm class:

Because the engineer placed the detection logic in OB40, and OB40 is not bound to any PROFINET alarm source, the OB is never scheduled. The MOVE inside the OB therefore never runs.

4. Required Configuration of OB82

OB82 is supported on every S7-1200 CPU whose firmware is V3.0 or newer. The OB is generated through the project tree:

1. Project tree → PLC_1 → Program blocks → Add new block → Organization block → Diagnostic error interrupt (OB82).
2. Open the OB82 editor; the local temp variables LADDR, IOSTS, Channel, MultiError, EventClass, FaultId, EvId are pre-declared.
3. Implement the required MOVE (or function call) in the OB body.
4. In every called function block (FB/FC) used inside OB82: open Block properties → Attributes and deselect "Handle errors within the block" (Handle errors locally = OFF). If "Local error handling" is enabled for an FB called by OB82, the diagnostic call chain stops at that block and OB82 is not re-entered.
5. Compile and download the project to the CPU in STOP, then switch back to RUN.

5. Reading PROFINET Diagnostic Data with SFB54 (RALRM)

To identify which device failed, read the diagnostic record from inside OB82 with the RALRM instruction (SFB54 on S7-1200 / S7-1500, identical interface):

// ST inside OB82
// Inputs
#RALRM_ID := 0;          // 0 = call from OB, 1..n = handle from previous call
#MODE := 1;              // 1 = read all output parameters
#F_ID := 0;              // not used when called from OB
#MLEN := 100;            // max length of diagnostic buffer
#NEW_PTR := FALSE;       // not used when called from OB
#TINFO := #tInfo;        // 34 bytes of task info
#AINFO := #aInfo;        // 34 bytes of ACK info
#LEN := #iLen;           // actual length returned
#STATUS := #wStatus;     // status word

IF #STATUS = 0 THEN
    // #tInfo.IO_STATE / #tInfo.IO_TYPE / #tInfo.LEN give channel info
    // #aInfo[0..3] is slot, #aInfo[4..5] is submodule ID, #aInfo[6..7] is alarm specifier
    // Evaluate #iLen bytes for the raw channel diagnostic record
    // AINFO[0] = ChannelNumber
    // AINFO[1..2] = ChannelProperties (bit 0..15)
    // AINFO[3..4] = ChannelErrorType
END_IF;

6. Hardware Interrupt OBs 40–47 — When They Are Right

If the requirement is genuinely a hardware interrupt (not a PROFINET diagnostic), then OB40 on S7-1200 is configured through the device view of the DI module, not through OB82.

From the TIA Portal help for hardware interrupt OBs, the available instructions for runtime control of OB40–OB47 are:

The runtime instruction set is identical on S7-1200 FW V4.2 and later. For earlier FW V4.0/V4.1, hardware interrupts are enabled by default; an SFC 30–33 call returns an "Instruction not supported" error and can be ignored.

7. Step-by-Step Conversion: OB40 → OB82

1. Open the project in TIA Portal V14.
2. Right-click Program blocks → Add new block → Organization block → Diagnostic error interrupt → confirm OB82.
3. Cut the MOVE (or call to a function) from the original OB40 and paste it into OB82.
4. Delete the obsolete OB40 from the project tree (right-click → Delete → confirm).
5. Open each FB/FC that the OB now calls. In Properties → Attributes uncheck "Handle errors within the block". Save and rebuild.
6. Place a watch-table tag of type Bool that you set inside OB82 (e.g. "PN_Fault_Active" := TRUE) so you can verify in online mode that the OB has been called.
7. Compile the project (Project → Compile all). Resolve any errors flagged in the messages window.
8. Download to the S7-1200 in STOP, switch to RUN.
9. In online & diagnostics, expand Online & Diagnostics → PROFINET interface → Diagnostics to see the live device list.

8. Commissioning Verification

1. Go online with the CPU (Online → Go online).
2. Open the watch table that monitors PN_Fault_Active.
3. Unplug the PROFINET device from the switch / cable. Within ≤ 200 ms the IO controller raises the diagnostic alarm.
4. Verify that PN_Fault_Active transitions to TRUE and that the yellow SF (system fault) LED on the CPU lights.
5. Reconnect the device. The alarm goes out, the SF LED clears, and OB82 is called a second time with the "incoming → outgoing" transition.
6. If PN_Fault_Active remains FALSE after a 2-second wait, see the troubleshooting matrix in section 10.

9. Fault Code Reference (PROFINET Diagnostic Alarms)

When a PROFINET device is unplugged, OB82 receives ChannelErrorType = 0x0100 with the slot / channel that maps to the IO device's interface submodule (slot 0, subslot 1 for the head module).

10. Troubleshooting Matrix

11. Standards and Documentation References

• PROFINET Conformance Class B / C diagnostic model — IEC 61784-2.
• PROFINET alarm model — IEC 61158-6-10 (Application Layer service "ALARM-ACK", "ALARM-PDU").
• SIMATIC S7-1200 Programmable Controller System Manual (Siemens).
• TIA Portal Help: Hardware Interrupt OBs OB 40–47.

12. Safety and Design Considerations

The OB82 priority is set in Properties → Attributes → Priority. Default priority 9 on S7-1200 is sufficient for diagnostic handling; raising it above the cyclic OB1 priority (1) does not interfere with PROFIsafe execution. Keep OB82 short — typically < 1 ms — to avoid jitter on time-critical alarms. If heavy processing is required, set a flag and process the diagnostic payload inside OB1.

Frequently Asked Questions