120400

Access PLC Diagnostics

Navigate to HMI → Diagnostics → PLC → Status to view current PLC state and error buffer.

Check timestamp of last error to correlate with machine events

Analyze Error Buffer

Review PLC error buffer for specific fault codes. Look for OB121 (programming errors) or OB122 (I/O access errors) entries.

Do not clear error buffer until root cause is identified

Check Program Logic

Use STEP 7 or TIA Portal to examine the PLC program around the fault address. Verify data types, array bounds, and mathematical operations.

Enable program monitoring to trace execution path leading to error

Restart PLC

After correcting program logic, set PLC to RUN mode via HMI → Machine → PLC or programming device.

Ensure machine is in safe state before restarting PLC

Check Hardware Configuration

Navigate to HMI → Diagnostics → Hardware to view I/O module status. Red indicators show faulty modules.

Note the slot number and module type for replacement ordering

Verify Module LEDs

Physically inspect I/O modules for red error LEDs or missing green RUN indicators on the control cabinet.

Follow LOTO procedures before opening electrical panels

Test Module Communication

Use STEP 7 hardware diagnostics to ping individual modules and check PROFIBUS/PROFINET communication status.

Check terminating resistors on PROFIBUS networks if multiple modules are offline

Replace Faulty Module

Power down system, replace defective I/O module, and download hardware configuration if module address changes.

Verify module part number matches HW Config before installation

Check HMI Stop Status

Navigate to HMI → Machine → PLC to verify if STOP was initiated locally through operator interface.

Check user log to identify who initiated the stop command

Verify External Stop Inputs

Check PLC input status for external STOP signals from maintenance switches or safety circuits in I/O diagnostics.

Ensure all safety conditions are satisfied before restart

Confirm Programming Device

Check if STEP 7 or TIA Portal is connected and verify PLC was not stopped remotely for programming activities.

Disconnect programming devices to prevent accidental stops during production

Restart PLC Operation

Set PLC to RUN mode via HMI → Machine → PLC → Start or programming device after confirming stop reason.

Verify machine is in safe starting position before PLC restart

Check Power Supply LEDs

Verify 24VDC power supply module shows green OK LED and no red fault indicators in the control cabinet.

Check load percentage on power supply display if available

Measure Supply Voltages

Use multimeter to verify 24VDC supply voltage at PLC CPU terminals is within 20.4-28.8VDC range per SIEMENS specifications.

Use proper PPE and follow electrical safety procedures

Check Power Consumption

Calculate total I/O module current draw and verify power supply capacity is not exceeded. Check for short circuits on output modules.

Add 20% safety margin to power supply sizing calculations

Restart After Power Stabilization

Once power issues are resolved, clear PLC memory if required and restart via HMI → Machine → PLC → Start.

Monitor voltage during restart to ensure stable operation

Check Battery Status

Navigate to HMI → Diagnostics → PLC → Memory to view backup battery voltage and memory card status indicators.

Battery voltage below 2.7V indicates replacement needed

Verify Memory Card

Check MMC/SD card is properly seated in CPU slot and not write-protected. Verify card capacity and format compatibility.

Never remove memory card while PLC is powered

Reload PLC Program

Download complete PLC program from STEP 7 project to CPU memory and memory card to restore proper operation.

Always maintain backup copies of PLC programs with version control

Replace Battery if Required

Power down system and replace backup battery following SIEMENS CPU manual procedures for your specific CPU model.

Use only SIEMENS approved battery types to prevent damage