Code 5311: FSSB: ILLEGAL CONNECTION
Issue Description & Safety Warning
The Fanuc 16i/18i/21i CNC has triggered a critical FSSB: ILLEGAL CONNECTION 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
1. This alarm is issued if, in a pair of axes in which one axis has an odd servo axis number (parameter No. 1023) and the other has an even servo axis number that is adjacent to the odd servo axis number, one of the axes is assigned to an amplifier connected to an FSSB in a system different from that for the other axis. 2. This alarm is issued if the system does not satisfy a constraint for performing high–speed HRV control, current control periods for two FSSBs are different, and it is specified that pulse modules connected to an FSSB in different paths are to be used. ??? BP/S alarm BP/S alarm occurs in the same number as the P/S alarm that occurs in ordinary program edit. (P/S alarm No. 070, 071, 072, 073, 074, 085 to 087) Modify the program.
Source: Fanuc Series 16i/18i/21i-B Operator's Manual, Appendix G Alarm List (B-63864EN/03)
Last reviewed: 2026-07-01 · Verified against the manufacturer’s official documentation.
About Fanuc 16i/18i/21i CNC Alarms
Fanuc controllers include extensive self-diagnostics that flag or halt the machine when a fault is detected. Error code 5311 on the 16i/18i/21i CNC identifies one specific fault condition — its documented cause and the official corrective steps are listed above, sourced from Fanuc's service documentation. Always confirm against the manual for your exact machine configuration and follow LOTO procedures before servicing.
Complete Alarm List: 360 Fanuc 16i/18i/21i CNC codes.
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