Serial encoders sit at the heart of every Yaskawa Sigma-V (SGDV) and Sigma-7 (SGD7S) servo axis. When the encoder loses power, its memory, or its connection to the SERVOPACK, the drive shuts down immediately with an A.8xx or A.Cxx alarm — and production stops with it. These faults are among the most common Yaskawa servo alarms we see at Test Well Electronics, and most of them trace back to just three root causes: an expired battery, a damaged or noisy encoder cable, or an encoder that has reached the end of its life.
If you keep Yaskawa servo drives and motors in stock or repair them in-house, this guide walks you through each encoder alarm code, what it actually means inside the encoder, and how to fix it step by step. For replacement drives and motors, browse our Yaskawa encoder and servo parts collection.
| Alarm | Name | What the drive detected | Resettable? |
|---|---|---|---|
| A.810 | Encoder Backup Error | All encoder power supplies failed; multi-turn position data was cleared | No — requires Fn008 setup |
| A.820 | Encoder Checksum Error | Checksum of encoder memory is incorrect | No |
| A.830 | Absolute Encoder Battery Error | Battery voltage below 2.7 V specification | Yes (Gr.1) |
| A.840 | Encoder Data Error | Data read from the encoder is incorrect | No |
| A.850 | Encoder Overspeed | Shaft rotating above 200 min-1 when power turned ON | No |
| A.860 | Encoder Overheated | Internal encoder temperature too high | No |
| A.C80 | Absolute Encoder Clear Error / Multiturn Limit Error | Multi-turn data not properly cleared or set (Fn008/Fn00B) | No |
| A.C90 | Encoder Communications Error | SERVOPACK cannot communicate with the encoder | Yes (Gr.1) |
A.830 Battery Error — the most common one. The drive powers up normally but the alarm appears within seconds, and the panel or the digital operator shows A.830. The encoder backup battery has dropped below the 2.7 V threshold the SERVOPACK monitors. Causes: the battery is simply old (Yaskawa rates the battery for roughly two years of service, less in hot cabinets), the battery connector was never plugged in after encoder or cable work, or the battery wiring is damaged. In rare cases the SERVOPACK’s battery charging circuit is at fault, not the battery itself.
A.810 Backup Error. This one is more serious: the encoder lost all power at some point — both the +5 V supply through the encoder cable and the battery — so the absolute multi-turn position data was wiped. Typical causes: the machine sat powered-off for months with a dead battery, the encoder cable was disconnected while the control power was off but the battery was also missing, or a bare encoder was stored for too long before installation.
A.820 Checksum and A.840 Data Errors. The encoder’s internal memory is failing or being corrupted. Noise is a frequent contributor here: encoder cables routed parallel to motor power cables or VFD output cables, missing shield grounding, or a damaged cable with intermittent contacts. If A.820 or A.840 returns even after a clean re-setup, the encoder electronics themselves have failed and the motor (or encoder unit) needs replacing.
A.850 Encoder Overspeed. The drive detected the shaft spinning faster than 200 min-1 at the moment control power came ON. Something is turning the motor while the drive is off: gravity on a vertical axis with a failed brake, windmilling fans, stored energy in a counterweight, or a mechanical back-drive condition. It can also indicate an encoder fault, but check mechanics first.
A.860 Encoder Overheated. Ambient temperature above 40°C around the motor, sustained overloading (check the load ratio monitor Un009), or a failing encoder.
A.C80 Clear/Multiturn Limit Error. Appears after incorrect use of Fn008 (absolute encoder clear) or Fn00B (multi-turn limit setting), or when the multi-turn data is corrupted. Usually procedural rather than a hardware failure.
A.C90 Communications Error. The SERVOPACK and encoder simply cannot talk. Causes: loose or corroded connectors at CN2, broken encoder cable conductors, severe electrical noise, or a failed encoder.
Step 1 — Read the alarm history first. Use the digital operator (or Yaskawa’s SigmaWinPlus) to check which alarm occurred, when, and how often. A single A.830 after a long shutdown points straight at the battery; repeated A.C90 during spindle operation points at cable noise.
Step 2 — For A.830: replace the battery with the machine powered up. This is the critical detail. With control power ON, replace the battery at the SERVOPACK (JZSP-BA01 type for most SGDV/SGD7S drives — confirm the part number for your drive size). Never replace the battery with the power off: that creates the exact condition that triggers A.810. Measure the old battery; anything under 2.7 V confirms the diagnosis. If A.830 returns within days with a fresh battery, suspect the SERVOPACK’s battery circuit.
Step 3 — For A.810/A.C80: re-establish the absolute reference. After restoring battery power, clear and re-set up the encoder using utility Fn008 (and Fn00B for the multi-turn limit), then re-home the machine according to your machine builder’s procedure. If Fn008 setup fails repeatedly, the encoder is faulty.
Step 4 — For A.820/A.840: eliminate noise, then re-test. Separate encoder cabling from power cabling, verify the shield is grounded at the drive end, and check connectors for corrosion. Re-run Fn008 setup. If the checksum error persists on a clean installation, replace the encoder or motor.
Step 5 — For A.850: secure the axis. Investigate what moved the shaft at power-off. Test the holding brake, release counterweights, and check that the axis cannot back-drive. Only after the mechanical cause is fixed should you cycle power again.
Step 6 — For A.C90: work the connection end to end. Reseat CN2 at both the motor and the SERVOPACK. Inspect the cable for crushed or flexed sections — encoder cables in cable chains have a finite flex life. Swap in a known-good cable if available. Ensure encoder wiring runs in its own conduit away from motor power leads. If a known-good cable still gives A.C90, the encoder’s communication stage has failed.
A battery or cable fix is always worth doing on site. But when the encoder electronics themselves have failed (persistent A.820/A.840/A.C90 with clean wiring), replacing the encoder — or the whole motor — is usually the faster and more reliable option. Encoder replacement on Yaskawa servos is not a generic swap: the serial encoder must match the motor and drive family, and the new unit must go through the absolute-setup procedure. If your SGDV or SGD7S drive itself is damaged (a failing SERVOPACK can also cause encoder-side alarms), we stock replacement drives such as the Yaskawa SGD7S-470A20A and SGDH-60AE-P, tested and ready to ship — see the full range in our servo drive collection.
Q: Can I replace the Yaskawa encoder battery with the machine off?
A: Avoid it. With both the +5 V supply and battery absent, the encoder loses its absolute data and you will face A.810 and a full Fn008 re-setup. Replace the battery with control power ON.
Q: A.810 appeared after a long shutdown. Is the encoder dead?
A: Usually not. Restore a good battery, power up, run Fn008 setup, and re-home. If setup repeatedly fails, then the encoder has genuinely failed.
Q: Are Yaskawa encoders interchangeable between motor models?
A: No. Serial encoders are matched to the motor and drive series (Sigma-V vs Sigma-7, absolute vs incremental). Always replace with the same encoder specification and redo the absolute setup.
Q: The alarm shows A.C90 but the cable looks fine. What next?
A: Reseat both connectors, try a known-good cable, and check noise sources such as unfiltered VFD output cables running alongside the encoder line. If the fault follows the motor and not the cable, the encoder has failed.
By Jennifer Zhang — Test Well Electronics supplies tested Yaskawa servo drives, motors, encoders and other industrial automation spare parts worldwide. Contact us for a quote.
