Code 320-0420: PLC: index range incorrect
Issue Description & Safety Warning
The Heidenhain TNC Series CNC has triggered a critical PLC: index range incorrect 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
[Cause of error] PLC runtime error: - The address for a writing access to data types B/W/D/M/ I/O/T/C is, through the inclusion of the index register, in an invalid region for these operand types. - The index register contains a value, due to accessing a constant field, which is not possible for this field (less than 0, or greater than or equal to field length). - The address of a string leads through the inclusion of the index register to an invalid value. - The number of a dialog (S#Dn[X]) or an error message (S#En[X]) leads through the inclusion of the index register to an invalid value (less than 0 or greater than 999). - During the addressing of a component string. [Error correction] Edit the PLC program.
Source: HEIDENHAIN NC Error Messages — TNC7/640/620/320/128, CNC PILOT 640, MANUALplus 620 (10/2023)
Last reviewed: 2026-07-01 · Verified against the manufacturer’s official documentation.
About Heidenhain TNC Series CNC Alarms
Heidenhain controllers include extensive self-diagnostics that flag or halt the machine when a fault is detected. Error code 320-0420 on the TNC Series CNC identifies one specific fault condition — its documented cause and the official corrective steps are listed above, sourced from Heidenhain's service documentation. Always confirm against the manual for your exact machine configuration and follow LOTO procedures before servicing.
Complete Alarm List: 4497 Heidenhain TNC Series CNC codes.
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