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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.