Servo encoder communication error
An encoder communication error means the servo amplifier cannot read position data from the motor encoder, so it stops the motor. Every manufacturer’s first suspects are the same: the encoder cable and connectors, the cable shield and routing (noise), a wrong cable or communication setting, then the amplifier or encoder itself, which a swap test separates.
Diagnose this with AI
Try Free Servo Diagnostic Upload a photo of the alarm screen, the fault history or a data export. InduQT compares the evidence and suggests the next useful checks.
Where it shows up
- Yaskawa Σ-7: A.C90 Encoder Communications Error (also A.C91, A.C92)
- Mitsubishi MR-J4: AL.16 Encoder initial communication error 1, AL.20 Encoder normal communication error 1
- Mitsubishi MR-J5: AL.016 Encoder initial communication error 1, AL.020 Encoder normal communication error 1
- Allen-Bradley Kinetix 5700: FLT S43/S44 FDBK LOSS, FLT S45 FDBK COMM FL, INIT FLT M03 MTR ENC STARTUP
- FANUC αi with CNC: serial data error 368 and data transfer error 369 (built-in Pulsecoder)
Likely causes and how to tell them apart
1Encoder cable open, shorted or with a bad connector
- Points to it
- Alarm at every power-up, or in motion when the cable flexes; after a motor or cable change.
- Check
- With power off, check continuity end to end and inspect both connectors for bent, wet or corroded pins.
2Noise: shield not terminated, cable run beside motor power
- Points to it
- Random alarms, more with nearby drives or welders switching.
- Check
- Check the shield is bonded at the connector ground plate and separate the encoder cable from the motor power cable.
3Wrong cable or communication setting
- Points to it
- New installation, long or third-party cable.
- Check
- Confirm the cable is the maker’s specified type and length (Yaskawa: 50 m max for rotary motors) and that the communication method parameter matches (MR-J4 PC04/PC22, MR-J5 PC04.3/PC22.3).
4Encoder (motor) failure
- Points to it
- Alarm follows the motor to another amplifier.
- Check
- Swap test: connect the motor to a known-good amplifier of the same type.
5Amplifier failure
- Points to it
- Alarm stays with the amplifier when a good motor is connected.
- Check
- Same swap test in reverse; replace the amplifier if the alarm stays with it.
Diagnostic sequence
- Record the exact codeNote the code and any detail or sub-code (for example AL.16.3, S43 sub-code). The detail number often names the cause.
- When does it happen?Every power-up points to a connector, open conductor or setting. Only in motion points to the cable in a carrier or a vibrating connector. Random points to noise and shielding.
- Inspect and test the cablePower off, lock out, then check connectors and continuity, flexing the cable along its run.
- Check shield and routingConfirm shield termination and separation from motor power cables.
- Swap testMove the motor to a known-good amplifier, or a good motor to the suspect amplifier, to decide which part goes for repair.
Evidence to collect
- Exact alarm code with detail number
- When it occurs: power-up, servo-on, motion, random
- Encoder cable part number, length and route
- Photos of connectors and shield termination
- Swap test result
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.