Yaskawa Σ-7 (Sigma-7) SERVOPACK alarm A.C90: Encoder Communications Error
Alarm A.C90 on a Yaskawa Σ-7 SERVOPACK is Encoder Communications Error: communications between the encoder and the SERVOPACK are not possible. It is a Gr.1 alarm and cannot be cleared with an alarm reset until the cause is removed.
Diagnose this with AI
Try Free Servo Diagnostic Upload a keypad photo, the fault history or a parameter export from your Yaskawa Σ-7 (Sigma-7) SERVOPACK. InduQT compares the evidence and suggests the next useful checks.
When it happens
- At power-up, when the SERVOPACK first tries to talk to the encoder (connector, wiring or encoder fault)
- During operation or motion, when vibration or cable flexing breaks contact, or noise corrupts the data
- After the motor, cable or SERVOPACK was replaced or reconnected
Likely causes and checks
1Faulty contact or wrong wiring at the encoder connector
- Points to it
- Alarm at power-up, often right after a motor or cable change.
- Check
- Check the condition of the encoder connector at both ends, reconnect it and check the encoder wiring.
2Encoder Cable open, shorted, or outside the specified impedance
- Points to it
- Alarm repeats at power-up or comes and goes with cable movement (cable carrier, robot dress pack).
- Check
- With power off, check the Encoder Cable for continuity and shorts and confirm it is a cable within Yaskawa’s specifications (shielded twisted pair, 50 m maximum for rotary motors).
3Environmental damage: corrosion, water or cutting oil in the connector, or contact loosened by vibration
- Points to it
- Machine tool or washdown area; green or wet pins; alarm appears during machining vibration.
- Check
- Inspect the connectors and operating environment; improve the environment and replace the cable. If the alarm continues, Yaskawa says to replace the SERVOPACK.
4Noise on the encoder signal
- Points to it
- Random alarms, more frequent when other large loads switch; encoder cable run beside the motor power cable.
- Check
- Separate the Encoder Cable from the Servomotor Main Circuit Cable and ground the encoder correctly, then see if the alarm stops.
5SERVOPACK or encoder (motor) failure
- Points to it
- Wiring and environment are good and the alarm is persistent.
- Check
- Swap test: connect the Servomotor to another SERVOPACK and turn on control power. No alarm means the SERVOPACK is faulty; alarm follows the motor means the motor encoder is faulty.
6Reserved parameter Pn0D8 changed from its default
- Points to it
- Parameters were copied from another machine or edited.
- Check
- Initialize the parameter settings (after saving a backup) as Yaskawa directs.
Telling it apart
- A.C90 means no communication at all; A.C91 is a position data acceleration-rate error (noise or bent cable while running) and A.C92 is a communications timer error. A.C91/A.C92 point more toward noise, vibration and shock than toward an open conductor.
- If the alarm only appears with the axis moving, suspect the cable in the carrier or a vibrating connector; if it appears every time at power-up, suspect a connector, an open conductor or the encoder itself.
- The swap test with another SERVOPACK is the quickest way to decide whether the drive or the motor goes for repair.
Relevant parameters
| Parameter | Name | Why it matters |
|---|---|---|
| Pn0D8 | Reserved parameter | Yaskawa lists a non-default value here as a cause of A.C90; initialize parameters to correct it. |
Evidence to collect
- Alarm history showing when A.C90 occurred and whether A.C91/A.C92 also appear
- Encoder Cable part number and length
- Photos of both connectors
- Result of the swap test with another SERVOPACK
- Whether the cable runs in a carrier or alongside the motor power cable
Work on fuel trains, burners, pressure vessels and electrical equipment must be done by qualified personnel following the manufacturer’s documentation and local codes. Never bypass a safety device or interlock.