What the A.90 alarm means

Serial communication between the drive and the encoder failed; the drive has no position feedback and blocks servo-on for the axis. This is a fault we handle frequently on ESTUN ERC / ED3L controller + ESTUN servo systems — in most cases we can pin down the root cause on the first site visit.

Most common causes

  • Encoder cable broken inside the cable track
  • Loose or oxidized encoder connectors
  • Electrical noise on the encoder line
  • Faulty encoder or drive communication circuit
Field note: The first item on this list is what we hit most often on real machines — clear that one before going deeper.

Step-by-step field fix

  1. Reseat encoder connectors at motor and drive ends
  2. Continuity-test the cable, especially through the dress-pack bend points
  3. Separate encoder wiring from motor power cables and check the shield
  4. Swap-test with a known-good cable
  5. Replace the encoder (motor) or drive based on the swap result
Safety first: For ESTUN a-90, 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. The self-serviceable action is Reseat encoder connectors at motor and drive ends; anything past that falls under position feedback and brake release and needs a certified engineer.

How we diagnose encoder alarms on site

ESTUN a-90 belongs to the encoder & position family. On ESTUN a-90 the failing stage is absolute encoders with battery on the ESTUN robot controllers (motion-control-platform based) - the feedback path: encoder body, encoder cable, battery, measuring board and the stored position memory. We start reading ESTUN a-90 at the pendant alarm list (A.xxx / E.xxx segments) and the servo panel display code.

The chain involved in ESTUN a-90 runs: motor-side encoder -> encoder cable -> drive / measuring board acquisition -> position count and zero data -> battery-held memory.

If ESTUN a-90 shows Encoder cable broken inside the cable track and tracks arm pose, the hard case arrives: Encoder cables flexing in a drag chain fatigue their copper cores: open at one pose, conducting at another. This is the nastiest to reproduce because it is intermittent. We jog the suspect axis near the pose recorded at alarm time and measure continuity while moving, watching the feedback for jumps. If the cable in that chain is past its rated bend count, we recommend replacing the whole run rather than splicing, even when this repair holds.

Field notes on this page cover ESTUN a-90 (Encoder Communication Error); written by the Henghuan repair team, updated 2026-10-01.

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Related Encoder & Position alarms

A.81 — Encoder Backup Error — Absolute Position Lost A.83 — Encoder Battery Voltage Low
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 INOVANCE · EFORT · JAKA and more — see the complete alarm code hub.

Frequently asked questions

QWhat does the ESTUN a-90 alarm mean?

Serial communication between the drive and the encoder failed; the drive has no position feedback and blocks servo-on for the axis. This is a fault we handle frequently on ESTUN ERC / ED3L controller + ESTUN servo systems — in most cases we can pin down the root cause on the first site visit. ESTUN a-90 sits in the encoder & position family; on our work orders the highest-frequency trigger for this code is "Encoder cable broken inside the cable track".

QCan I clear a ESTUN a-90 alarm myself?

Whether ESTUN a-90 clears on its own depends on the outcome of "Reseat encoder connectors at motor and drive ends". Finish the flow with "Replace the encoder (motor) or drive based on the swap result" to confirm ESTUN a-90 - do not skip it. If that step is clean, the second most frequent cause on ESTUN a-90 is "Loose or oxidized encoder connectors". Two attempts and still there means stop: ESTUN a-90 involves position feedback and brake release, and running ESTUN a-90 on can turn a repairable fault into a replacement.

QFor encoder alarms like ESTUN a-90, can it be fixed in one visit?

The commonest causes we see on ESTUN a-90 are "Encoder cable broken inside the cable track, Loose or oxidized encoder connectors, Electrical noise on the encoder line", so we stock the van against those. For ESTUN a-90 specifically, we carry feedback-path parts: encoder cables, battery packs, connector pins and the measuring-board type data for the brand. For ESTUN a-90, 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 ESTUN a-90 shows on the pendant alarm list (A.xxx / E.xxx segments) and the servo panel display code and what the machine was doing - phone or video triage is free.

This page on ESTUN a-90 was written by the Henghuan Robotics repair team from field service experience. Alarm descriptions are our own summary for troubleshooting reference and may vary by controller and software version — when in doubt, check your ESTUN documentation or contact us. Henghuan is an independent service provider, not affiliated with or endorsed by ESTUN.