Code 17020: [Channel %1: ] Block %2 illegal array index 1
Issue Description & Safety Warning
The Siemens SINUMERIK 840D CNC has triggered a critical [Channel %1: ] Block %2 illegal array index 1 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 = Channel number %2 = Block number, label Definitions: General: A read or write access has been programmed to an a rray variable with an invalid 1st array index. The valid array indices must lie within the defined array size and the absolute limits (0 - 32,766). PROFIBUS I/O: An invalid slot / I/O area index was used while reading/writing data. Cause: 1.: Slot / I/O area index >= max. number of available slot / I/O areas. 2.: Slot / I/O area index references a slot / I/O area that has not been configured. 3.: Slot / I/O area index references a slot / I/O area that has not been released for a system variable. The following applies specifically: If the alarm oc curs while writing from one of the parameters $TC_MDP1/$TC_MDP2/$TC_MLSR, check whether MD18077 $MN_MM_NUM_DIST_RE L_PER_MAGLOC has been set correctly. MD18077 $MN_MM_NUM_DIST_REL_PER_MAGLOC defines the number of different Index1 statements that may be made for an Index2 value. Reaction: Correction block is reorganized. Interface signals are set. Alarm display. Remedy: Correct the specification of array elements in t he access instruction to match the defined size. If an SPL is used in Safety Integrated, the field index via optional data may be subject to additional restrictions. Program Continuation: Clear alarm with NC ST ART or RESET key and continue the 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 17020 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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