Code 120403: Bus%1.Slave%2: %3: Check/acknowledge topology!
Issue Description & Safety Warning
The Siemens SINUMERIK 840D CNC has triggered a critical Bus%1.Slave%2: %3: Check/acknowledge topology! alarm. Immediate attention from a qualified operator or maintenance technician is required to prevent secondary damage to the equipment or spoiled parts during the manufacturing cycle. Always ensure standard safety and LOTO procedures are followed before accessing electrical cabinets.
Official Troubleshooting Steps
Parameters: %1 = Bus number %2 = Slave address %3 = Name of the affected drive unit Definitions: The drive unit with the bus number and slave address specified in the alarm has detected an illegal difference between reference topology and actual topology during the ramp-up when checking the DRIVE-CLiQ topology. For this reason, the drive unit has stopped t he ramp-up in the 'Topology error' state. Reaction: Alarm display. Remedy: - Check actual topology and possibly repl ug in line with the actual topology. - Check DRIVE-CLiQ cables for breakage and problems with contacts. - Test DRIVE-CLiQ components for operational functioning. Note: Under 'Commissioning > Drive system > Drive unit > Topology', HMI offers a suitable diagnosis (e.g. reference/actual value comparison). Program Continuation: Internal
Source: SINUMERIK 840D sl / 840Di sl / SINAMICS S120 Diagnostics Manual (DAsl, 03/2009)
Last reviewed: 2026-07-01 · Verified against the manufacturer’s official documentation.
About Siemens SINUMERIK 840D CNC Alarms
Siemens controllers include extensive self-diagnostics that flag or halt the machine when a fault is detected. Error code 120403 on the SINUMERIK 840D CNC identifies one specific fault condition — its documented cause and the official corrective steps are listed above, sourced from Siemens's service documentation. Always confirm against the manual for your exact machine configuration and follow LOTO procedures before servicing.
Complete Alarm List: 4014 Siemens SINUMERIK 840D CNC codes.
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