28033

Access Siemens HMI > Diagnostics > Alarms > Archive and note the exact NCU index (from %1 parameter). Verify which physical NCU in the network triggered the alarm.

In PLC program (OB1 or safety logic), verify emergency stop input signal status. Check physical E-stop button, safety relay module, and hardwired circuits to NCU control inputs.

Do NOT clear alarm until E-stop root cause is identified to prevent unintended axis motion.

Review MD 28040 (emergency stop behavior: 0=immediate stop, 1=follow-up mode) and MD 28041 (deceleration ramp) via SINUMERIK menu path: System > Parameters > MD [Emergency Stop]. Confirm values are appropriate for your application.

Clear the PLC E-stop signal by releasing physical E-stop button and verifying signal handshake returns to inactive state. Monitor PLC-NCK interface telegram for E-stop bit (typically in NCI/NCO interface blocks) to confirm transition.

Access SINUMERIK > System > Network > NCU Link (or PROFIBUS) diagnostics to identify communication errors or retry counts for the NCU listed in alarm %1. Check for corrupted frames or CRC errors.

Verify PLC-NCK interface telegram structure in SPF/SPL program. Confirm that the emergency stop request signal occupies the correct bit position in the NCK interface input telegram (typically bit 0 or bit defined in MD 28030-28035 Emergency Stop Parameter range).

Check cable integrity and termination resistors on PROFIBUS/Industrial Ethernet segment connected to the affected NCU. Use a digital multimeter or network analyzer to verify signal levels and absence of reflections.

Reload and recompile the PLC program (OB1, safety blocks) and download to the control system. Reset all NCUs via Siemens HMI > System > Reset > Warm Start to re-establish clean PLC-NCK coupling.

Power down the control system and inspect the physical terminal block or safety relay module connected to NCU emergency stop input. Look for corroded contacts, arcing damage, or loose wire ferrules on the affected NCU's input terminal.

Follow lockout/tagout procedures before any hardware inspection.

Measure DC voltage at the NCU E-stop input terminal (typically 24VDC logic level) with a digital multimeter while the system is powered. Active E-stop should read ~24VDC; inactive should read ~0VDC. If voltage does not change when E-stop button is released, suspect stuck relay or short circuit.

Check the internal pull-up resistor network (typically 4.7kΩ to 10kΩ) on the NCU input module. Measure resistance between the E-stop input terminal and 24VDC supply rail. Out-of-spec resistance indicates defective input circuit requiring NCU module replacement.