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FANUC Controller
Program/G-code · Intermediate · 15-30 minutes
Fanuc Alarm 221 - ILLEGAL COMMAND IN SYNCHRO-MODE
Fanuc Alarm 221 (ILLEGAL COMMAND IN SYNCHRO-MODE) occurs when a move command is issued to a synchronous control axis via the NC program or PMC axis control interface during active synchronous operation. The slave axis cannot accept independent motion commands while synchronous control is active. Fix by modifying the program to avoid commanding the synchronous slave axis directly, or cancel synchronous control before issuing the move.
221
Fanuc Alarm 221 - ILLEGAL COMMAND IN SYNCHRO-MODE
ILLEGAL COMMAND IN SYNCHRO-MODE
Fanuc PS221: Same as PS220 — an illegal command was detected in synchronous control mode. The specific sub-error indicates a different restricted function was attempted.
Root Cause Summary
Fanuc Alarm 221 (ILLEGAL COMMAND IN SYNCHRO-MODE) occurs when a move command is issued to a synchronous control axis via the NC program or PMC axis control interface during active synchronous operation. The slave axis cannot accept independent motion commands while synchronous control is active. Fix by modifying the program to avoid commanding the synchronous slave axis directly, or cancel synchronous control before issuing the move.
Safety
- Part ejection affected
Causes and Fixes
- 1
Identify the Synchronous Axis Pair
Check parameter No. 8180 (synchronous control axis assignment) to identify which axes are in master-slave synchronous relationship.
Document axis assignments before troubleshooting.
- 2
Review the NC Program
Find the block generating the alarm. Check if it commands motion on the slave axis. Remove or modify the command so only the master axis is addressed.
Use the diagnostic screen to identify which block triggered the alarm.
- 3
Cancel Synchronous Control if Needed
If independent slave axis motion is required, cancel synchronous control first using the appropriate synchronous control axis selection signal (PMC), then command the move.
Ensure synchronous control is re-enabled after the independent move if required.
Canceling synchronous control while axes are in motion can cause mechanical damage.
Call a technician if:
- ·If position sensor replacement or motor repair does not resolve the issue after thorough testing.
- ·If control system errors persist after reset and software update attempts.
Prevention
- Regularly clean the parts catcher assembly to prevent debris buildup on sensors and mechanical parts.
- Schedule routine maintenance checks on motors and interlock switches to catch wear early.
- Use proper shutdown procedures to avoid control system glitches.
Common Mistakes
- Ignoring debris buildup as a potential cause, leading to repeated sensor or mechanical failures.
- Bypassing interlock switches to temporarily clear the alarm, which compromises safety and violates safety regulations.
- Working on energized equipment without proper LOTO procedures.
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