Mitsubishi MR-J4 AL.20: Encoder normal communication error 1 while running, what to check?
I'm getting an AL.20 alarm on my Mitsubishi MR-J4 servo amplifier. It happens while the machine is running, and sometimes clears if I wiggle the encoder cable. What should I check first?
The AL.20 alarm on a Mitsubishi MR-J4 servo amplifier indicates an encoder normal communication error 1, meaning an error occurred in the communication between the encoder and the servo amplifier after start-up. This typically happens during operation, especially when the encoder cable flexes, or during acceleration/deceleration when electrical noise is higher. The fact that it sometimes clears when the cable is moved strongly points to a physical issue with the cable.
Likely causes and checks
1Encoder cable failing (open or shorted conductor)
- Points to it
- An open or shorted conductor in the encoder cable can disrupt communication. The alarm occurring during motion and clearing when the cable is moved is a strong indicator of a failing cable, especially if it's flexing in a cable carrier.
- Check
- Check the encoder cable for disconnection or shorts, flexing it along its run. Repair or replace the cable as needed.
2Cable shield not connected to the connector ground plate
- Points to it
- If the external conductor (shield) of the encoder cable is not properly connected to the ground plate of the connector, it can lead to communication errors due to noise, especially in environments with VFDs or welders. This often manifests as random alarms.
- Check
- Check that the external conductor of the encoder cable is securely connected to the ground plate of the connector.
3Noise, temperature or vibration near the device
- Points to it
- Excessive electrical noise, high ambient temperature, or significant vibration can interfere with encoder communication. If the alarm correlates with other equipment switching or machine vibration, these factors could be contributing.
- Check
- Check for sources of noise, ambient temperature, and vibration near the servo amplifier and encoder, and implement appropriate countermeasures.
4Wrong communication method setting
- Points to it
- Incorrect communication method settings can prevent proper communication between the encoder and amplifier. This is more likely after commissioning or a cable change.
- Check
- Check the communication method setting in [Pr. PC22] ([A]) or [Pr. PC04] ([B] [WB] [RJ010] [GF]).
Next check: Given that the alarm occurs during motion and can be influenced by moving the cable, the very first step should be a thorough inspection of the encoder cable for any physical damage, disconnections, or shorts, especially along its flexible sections. If the cable appears intact, verify the shield connection at the connector.
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