Siemens SIMATIC S7-1500 troubleshooting
The S7-1500 reports errors as events. Each event can call an error organization block (OB) if one is configured; if none is, the CPU applies a default system reaction. On the S7-1500 most error events are ignored by default and the CPU stays in RUN; only a programming error without OB121 (or local error handling) and a cycle time overrun without OB80 put the CPU in STOP, and a second overrun in the same cycle always does. This differs from S7-300/400, where missing error OBs generally stop the CPU.
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Try Free PLC Fault Diagnostic Upload a photo of the alarm screen, the fault history or a data export. InduQT compares the evidence and suggests the next useful checks.
How it reports faults
- Every CPU keeps a diagnostics buffer with internal and external module errors, CPU system errors, operating mode changes (e.g. RUN to STOP), user program errors and removal/insertion of modules, in the order they occurred. It survives a memory reset, so it is the first place to find the cause of a STOP.
- The diagnostics buffer can be read in STEP 7 (Online & diagnostics > Diagnostics buffer), on SIMATIC HMI devices, through the S7-1500 web server and on the CPU display.
- LEDs: RUN/STOP green = RUN; RUN/STOP yellow = STOP; ERROR flashing red = an error or a pending diagnostics event; MAINT yellow = maintenance demanded or an active force job; all off = missing or insufficient supply (CPU 1515-2 PN manual; check the manual for your CPU).
- Error OBs and default reaction if the OB is not configured: OB80 time error (STOP if the maximum cycle time is exceeded once without OB80; STOP always if it is exceeded twice in one cycle), OB82 diagnostic interrupt (ignore), OB83 pull/plug (ignore), OB86 rack error (ignore), OB121 programming error (STOP), OB122 I/O access error (ignore).
- Programming and I/O access errors can be handled locally in the block with GET_ERROR or GET_ERR_ID; then OB121/OB122 are not called. Siemens: to keep the CPU out of STOP, every programming and I/O access error must be caught by global or local error handling.
Fault codes
| Code | Name |
|---|---|
| OB80 | Time error (cycle time exceeded) |
| OB82 | Diagnostic interrupt |
| OB83 | Pull/plug interrupt (distributed I/O) |
| OB86 | Rack error (station or IO system failure) |
| OB121 | Programming error |
| OB122 | I/O access error |
Evidence to collect
- Diagnostics buffer export (Online & diagnostics), including entries before the STOP
- Photo of the CPU display and LEDs
- Online and diagnostics view of the faulted module or station
- TIA Portal project archive at the version loaded in the CPU
- CPU article number and firmware version
- Which error OBs (80, 82, 83, 86, 121, 122) exist in the program
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