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Mitsubishi MR-J4 AL.16 and AL.20 Alarms: Encoder Communication Errors

admin

AL.16 and AL.20 on Mitsubishi MR-J4 servo drives indicate encoder communication errors. These alarms are common on servo press applications due to the harsh electrical environment near high-power drives.

## AL.16 vs AL.20

- **AL.16**: Encoder initial communication error —occurs at power-on, encoder does not respond
- **AL.20**: Encoder normal communication error —occurs during operation, communication lost

Both indicate a problem with the serial communication between the drive and the encoder.

## Root Causes

**Cable issues (most common):**
- Encoder cable damaged —check for cuts, pinches, or excessive bending
- Connector not fully seated —reseat both ends firmly
- Cable too long —MR-J4 encoder cable maximum length is 50m (30m recommended)
- Cable routed near power cables —EMI interference

**Encoder issues:**
- Encoder contaminated with oil or coolant
- Encoder bearing failure causing internal shorts
- Encoder overheating (check motor temperature)

**Drive issues:**
- Drive encoder input circuit failure
- Power supply voltage to encoder out of range (check CN2 connector pin voltages)

## Diagnostic Steps

1. **Power cycle** —if AL.16 clears on retry, intermittent connection issue
2. **Swap encoder cable** with known-good cable —if alarm clears, cable is faulty
3. **Check cable routing** —ensure minimum 100mm separation from power cables
4. **Measure encoder supply voltage** —should be 5V ?5% at CN2 connector
5. **Check motor temperature** —encoder fails above 100?C ambient

## Cable Replacement

Use only Mitsubishi-specified encoder cables (MR-JCCBL series). Third-party cables often lack proper shielding for the high-frequency serial protocol used by MR-J4 encoders.

Shield must be connected at drive end only (single-point grounding) to prevent ground loops.

raj_maintenance

AL.16 and AL.20 on MR-J4 in press environments: the vibration is the root cause 80% of the time. The encoder connector on MR-J4 is not the most robust design. We add a small cable clamp 50mm from the connector to prevent the cable weight from pulling on the connector during vibration. This simple fix reduced our encoder alarm rate by 70%.

mike_chen_eng

Encoder communication issues are often the hardest to diagnose because they're intermittent. We had an AL.16 that would trigger randomly under load — turned out the encoder cable was routed too close to the servo motor power lines, causing EMI. Rerouting the cable with shielding solved it completely.

One diagnostic tip: if you see the alarm only under load or at high speed, it's almost always an EMI issue, not a hardware fault. Check your cable routing first before replacing components.

dave_toolmaker

For MR-J4 encoder alarms in high-vibration environments, also check the encoder mounting bolts. The standard M4 bolts can loosen over time. We apply medium-strength threadlocker on all encoder mounting bolts during installation. Re-check torque at the first quarterly PM. After that they usually stay put.