Siemens alarm 400551: Fault on MPI/DP bus
SINUMERIK PLC system alarm · Siemens 840D/810D powerline
What it means
The PLC detected an error on its MPI/PROFIBUS-DP I/O bus while operating distributed peripherals. The alarm is generic - it says only that I/O traffic on that bus is disturbed - so the concrete station has to be found from the bus diagnostics. Remedy per Siemens: check I/Os and rectify the I/O fault.
Likely causes
- Bus termination missing/faulty or segment interrupted
- Slave dropping off the MPI/DP segment
- EMC-induced telegram corruption (shielding, grounding)
- Wrong baud rate/address configuration after changes
Triage note: Program continuation: internal. See also 400552 (DP bus) and 400553 (PROFINET bus).
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Send us alarm 400551 with your machine details — typical RFQ turnaround is 24–48h.