Siemens alarm 120201: Communication failure to an HMI/OP/MCP/HT on internal MPI/PROFIBUS
SINUMERIK NCK/HMI communication alarm (PROFIBUS segment) · Siemens 840D/810D powerline
What it means
NCU lost handshake with a PROFIBUS-DP slave on operator segment (OP, machine control panel, handheld terminal).
Likely causes
- shield not making 360-degree saddle contact at connector backshell (single most common)
- missing bus terminator at last node
- 24 V supply missing for powered slave
- duplicate slave address
- defective HT6 handheld unit dragging the bus
Triage note: Standard isolation: disconnect HT6, then MCP, adding terminator at new last node each time; watch whether 400262/400264 disappear.
Replacement parts
We supply the following units for this alarm — tested, warranty-backed:
- Request a quote →
6XV1830-… PROFIBUS cable + 6ES7972-0… terminators
PROFIBUS cable & terminating resistor · Primary suspect
- Request a quote →
HT6 handheld unit (like-for-like, not HT2)
handheld terminal · Common cause
- Request a quote →
MCP machine control panel / OP unit
operator device (isolate next) · Check if others ruled out
Need the part urgently?
Send us alarm 120201 with your machine details — typical RFQ turnaround is 24–48h.