Mitsubishi Electric MELSERVO-J4 (MR-J4) alarm AL.20: Encoder normal communication error 1
Alarm AL.20 on a Mitsubishi MR-J4 servo amplifier is Encoder normal communication error 1: an error occurred in the communication between the encoder and the servo amplifier during normal communication, after start-up.
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When it happens
- While running, especially when the cable flexes in a carrier
- During acceleration or deceleration when motor current, and noise, is highest
- At random times when nearby equipment switches
Reading the subset
| Subset | What it tells you |
|---|---|
| 20.1–20.3 | Receive data error 1–3. |
| 20.5 | Transmission data error 1 (also check A/B-phase signals on A/B/Z linear encoders). |
Likely causes and checks
1Encoder cable failing (open or shorted conductor)
- Points to it
- Alarm during motion; may clear when the cable is moved.
- Check
- Check the encoder cable for disconnection or shorts, flexing it along its run; repair or replace it.
2Cable shield (external conductor) not connected to the connector ground plate
- Points to it
- Random alarms, worse near VFDs or welders; cable recently re-terminated.
- Check
- Check that the external conductor of the encoder cable is connected to the ground plate of the connector.
3Wrong communication method setting
- Points to it
- After commissioning or a cable change.
- Check
- Check [Pr. PC22] ([A]) or [Pr. PC04] ([B] [WB] [RJ010] [GF]).
4Amplifier or encoder failure
- Points to it
- Wiring, shield and settings check good.
- Check
- Replace the amplifier and check repeatability; then the servo motor or linear encoder.
5Noise, temperature or vibration near the device
- Points to it
- Alarm correlates with other equipment or vibration.
- Check
- Check noise, ambient temperature and vibration and take countermeasures.
Telling it apart
- AL.20 occurs in normal (running) communication; AL.16 in initial communication at start-up. An AL.20 that appears only in motion points to the cable run, shield termination or noise rather than a dead encoder.
- AL.21 (Encoder normal communication error 2) is a separate alarm for encoder data errors; record the exact number.
Relevant parameters
| Parameter | Name | Why it matters |
|---|---|---|
| PC22 ([A]) / PC04 ([B] [WB] [RJ010] [GF]) | Encoder cable communication method selection | Listed by Mitsubishi as a cause of AL.20. |
Evidence to collect
- Full alarm and detail number
- When it happens in the cycle
- Encoder cable route and shield termination photos
- Settings of [Pr. PC04]/[Pr. PC22]
- Swap test results
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