Yaskawa Sigma-7 A.C90 Alarm: Encoder Communications Error on Servo Press

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The A.C90 alarm (Encoder Communications Error) on Yaskawa Sigma-7 drives is particularly common on servo press applications due to the vibration environment.

Root Causes (in order of frequency):
1. Encoder cable damage: The cable flexes with every stroke. Check for cuts, pinching, or tight bend radii. Minimum bend radius: 10x cable diameter.
2. Connector contamination: Oil mist and metal particles get into the encoder connector. Clean with contact cleaner and apply dielectric grease.
3. Ground loop: If the encoder cable shield is grounded at both ends, a ground loop can cause intermittent errors. Ground at drive end only.
4. Vibration-induced connector loosening: Add a cable strain relief within 150mm of the motor on high-SPM presses.
5. Encoder failure: Run the encoder diagnostic in SigmaWin+ software.

Temporary Workaround: Reduce SPM by 30% and monitor. The alarm is often intermittent at lower speeds, giving you time to order the cable or encoder.

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Related reading: for more on this topic, see the discussion on [Yaskawa Sigma-7 A.810/A.820 alarm guide](https://servopress.club/d/53-53-yaskawa-sigma-7-drive-on-servo-press-a810-and-a820-alarm-troubleshooting) which covers complementary fault scenarios and diagnostic steps.