Code 25100: Axis %1 measuring system switchover not possible
Issue Description & Safety Warning
The Siemens SINUMERIK 840D CNC has triggered a critical Axis %1 measuring system switchover not possible 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 = Axis name, spindle number Definitions: The prerequisites are not satisfied for the required encoder switchover: 1. The newly selected encoder must be in the active state: (DB31,... DBX1.5 / 1.6 = 1 (Position measuring system 1/2). 2. The actual value difference between the two encoder s is greater than the value in the axis-specific MD36500 $MA_ENC_CHANGE_TOL ("Maximum tolerance for position actual value switchover"). Activation of the measuring system concerned takes place in accordance with the NC/PLC interface signals DB31,... DBX1.5 (Posit ion measuring system 1) and DB31,... DBX1.6 (Position measuring system 2), i.e. the position control is now operated with this measuring syst em. The other measuring system is switched over to follow-up mode. If both interface signals are set to "1", then only the 1st measuring system is active; if both interface signals are set to "0", the axis is parked. Changeover takes place as soon as the interface si gnals have changed, even if the axis is in motion. Reaction: NC Start disable in this channel. Interface signals are set. Alarm display. NC Stop on alarm. Remedy: Please inform the authorized personnel/service depar tment. When referencing the active position actual value encoder, the actual value system of the inactive encoder is set to the same reference point value as soon as phase 3 has been concl uded. A later positional difference between the two actual value systems can have occu rred only as the result of an encoder defect or a mechanical displacement between the encoders. - Check the encoder signals, ac tual value cable, connectors. - Check the mechanical fastenings (displacement of the measuring head, mechanical twisting possible). - Increase the axis-s pecific MD 36500 $MA_ENC_CHANGE_TOL. Program continuation is not possible. The program must be aborted with "RESET", then program execution can be reinitiated with NC START, if necessary at the interruption point after "Block search with/without calculation". Program Continuation: Clear alarm with the RESET key. Restart part program
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 25100 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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