What the A.C90 alarm means

Serial communication between the SERVOPACK and the encoder is impossible; the drive has no position feedback and refuses servo-on for that axis. This is one of the most common faults in our repair logs for the Yaskawa DX100 / DX200 / YRC1000 platform, and the first inspection typically locates the cause.

Most common causes

  • Damaged encoder cable (broken wire, crushed by cable track)
  • Loose or oxidized encoder connectors at motor or SERVOPACK
  • Electrical noise on the encoder line
  • Faulty encoder or SERVOPACK communication circuit
Field note: Field experience says the first cause is the most likely — verify it before moving on to the rest.

Step-by-step field fix

  1. Reseat the encoder connectors at both ends
  2. Continuity-test the encoder cable end to end, especially where it flexes in the cable track
  3. Check encoder cable separation from motor power cables and verify shielding
  4. Swap-test with a known-good cable
  5. Replace encoder (motor) or SERVOPACK based on the swap result
Safety first: For YASKAWA a-c90, encoder alarms often mean position data cannot be trusted: actual pose may disagree with the system record, so releasing a brake or jogging carries unexpected-motion risk. What you can check yourself first is Reseat the encoder connectors at both ends; position feedback and brake release handling is not a DIY job.

How we diagnose encoder alarms on site

YASKAWA a-c90 belongs to the encoder & position family. On YASKAWA a-c90 the failing stage is absolute encoders with a battery box; low battery voltage raises the encoder-battery alarms on the DX100 / DX200 / YRC1000 / YRC1000micro - the feedback path: encoder body, encoder cable, battery, measuring board and the stored position memory. We start reading YASKAWA a-c90 at the pendant alarm list (A / E / 4xxx segments) together with the servo drive's digit display.

The chain involved in YASKAWA a-c90 runs: motor-side encoder -> encoder cable -> drive / measuring board acquisition -> position count and zero data -> battery-held memory.

On YASKAWA a-c90 the commonest field report is Damaged encoder cable (broken wire, crushed by cable track); the highest-share cause is plain: The highest-share cause in this family is unglamorous: battery voltage. Absolute encoders hold the position count on a battery while power is off; below threshold the memory is gone and the controller demands a fresh master. Packs are designed for roughly two to three years, but a hot cabinet shortens that a lot. We carry spare packs, and after a swap we master immediately and re-check every axis zero so the customer does not run on an offset.

Field notes on this page cover YASKAWA a-c90 (Encoder Communications Error); written by the Henghuan repair team, updated 2026-10-01.

Blocked by this alarm? We can help you analyze it over the phone

Provide the alarm code, model and the action being performed at the alarm. Our engineers have seen YASKAWA a-c90 YASKAWA a-c90 many times and judge fast; phone and video remote diagnosis are both free.

Get a Free Remote Diagnosis See Repair Services

Hotline: +86-20-39029800 · WhatsApp: +86-188-1886-2860

Related Encoder & Position alarms

A.810 — Encoder Backup Error — Position Lost A.830 — Absolute Encoder Battery Error
Same family: this code belongs to the Encoder & position alarms family - the page collects all Encoder & position-family codes we have handled, with the common inspection order and spares.
Other brands: we also keep field-fix libraries for Universal Robots · Kawasaki · Staubli and more — see the complete alarm code hub.

Frequently asked questions

QWhat does the YASKAWA a-c90 alarm mean?

Serial communication between the SERVOPACK and the encoder is impossible; the drive has no position feedback and refuses servo-on for that axis. This is one of the most common faults in our repair logs for the Yaskawa DX100 / DX200 / YRC1000 platform, and the first inspection typically locates the cause. YASKAWA a-c90 sits in the encoder & position family; on our work orders the highest-frequency trigger for this code is "Damaged encoder cable (broken wire, crushed by cable track)".

QCan I clear a YASKAWA a-c90 alarm myself?

Whether YASKAWA a-c90 clears on its own depends on the outcome of "Reseat the encoder connectors at both ends". Finish the flow with "Replace encoder (motor) or SERVOPACK based on the swap result" to confirm YASKAWA a-c90 - do not skip it. If that step is clean, the second most frequent cause on YASKAWA a-c90 is "Loose or oxidized encoder connectors at motor or SERVOPACK". After clearing YASKAWA a-c90, watch it for one shift. A second trip means stop and let an engineer arrive with the right test gear for position feedback and brake release.

QFor encoder alarms like YASKAWA a-c90, can it be fixed in one visit?

The commonest causes we see on YASKAWA a-c90 are "Damaged encoder cable (broken wire, crushed by cable track), Loose or oxidized encoder connectors at motor or SERVOPACK, Electrical noise on the encoder line", so we stock the van against those. For YASKAWA a-c90 specifically, we carry feedback-path parts: encoder cables, battery packs, connector pins and the measuring-board type data for the brand. For YASKAWA a-c90, encoder alarms are mostly broken cores and dead batteries, both of which ride in the van, so most clear in one visit. Before we roll, tell us what YASKAWA a-c90 shows on the pendant alarm list (A / E / 4xxx segments) together with the servo drive's digit display and what the machine was doing - phone or video triage is free.

This page on YASKAWA a-c90 is based on the field experience of the Henghuan Robotics repair team. Alarm explanations are our own summary for troubleshooting reference and may differ across controller and software versions — if unsure, verify against Yaskawa documentation or reach out to us. Henghuan is an independent service provider with no affiliation or endorsement by Yaskawa.