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Code 201421: <location>Topology: Comparison different components

Issue Description & Safety Warning

The Siemens SINUMERIK 840D CNC has triggered a critical <location>Topology: Comparison different components 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

Message value: Component number: %1, component class target: %2, component class actual: %3, fault cause: %4 Drive object: All objects Reaction: NONE Acknowledge: NONE Cause: The topology comparison has detected differences in the actual and target topologies in relation to one component. The component class, the component type or the number of connections differ. Alarm value (r2124, interpret hexadecimal): ddccbbaa hex: aa = component number of the component, bb = component class of the target tolopology, cc = component class of the actual topology, dd = fault cause dd = 01 hex = 1 dec: Different component class. dd = 02 hex = 2 dec: Different component type. dd = 03 hex = 3 dec: Different Order No. dd = 04 hex = 4 dec: Different number of connections. Note: Component class, component type and connec tion number are described in F01375. The drive system is no longer booted. In this stat e, the drive control (closed-loop) cannot be enabled. Remedy: Check the component soft-wired connections against t he hardware configuration of the drive unit in the commissioning software and correct differences. Note: Under "Topology --> Topology view" the commissi oning software offers improved diagnostics capability (e.g. setpoint/actual value comparison).

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 201421 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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