Fanuc alarm 401: EXTERNAL BUS INVALID ADDRESS
30i/31i/32i system alarm · Fanuc 30i/31i/32i-B
What it means
A CNC bus master attempted to access an invalid address on the external (system) bus, indicating a problem on the inter-board bus of the multi-CPU 30i architecture. The main board or the MASTER/SLAVE PCB named on the alarm screen is the usual culprit, but external noise can produce identical symptoms on an otherwise healthy system.
Likely causes
- Defective main board, master PCB or slave PCB
- Poor grounding or nearby noise source inducing bus corruption
- Poorly seated option/sub-CPU board
Replacement parts
We supply the following units for this alarm — tested, warranty-backed:
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Main board / sub-CPU board as named on third line
board · Common cause
Need the part urgently?
Send us alarm 401 with your machine details — typical RFQ turnaround is 24–48h.