MR-J4 alarms 20.5 and 20.6: when to check A/B phases and when to check Z
For differential-output linear encoders, the 20.5 and 20.6 branches identify A/B-phase and Z-phase wiring checks. Read the Target column and the reference to 20.1 before applying those checks to another encoder interface.
Contents
Interpreting every MR-J4 AL.20.5 as an A/B wiring fault, or every AL.20.6 as a Z wiring fault, can send troubleshooting to the wrong interface. Mitsubishi Electric SH(NA)-030109ENG-T (November 2025), §1.4, printed page 42, explicitly makes these checks conditional on using an A/B/Z-phase differential-output linear encoder. This guide explains how to select the diagnostic branch using existing records and findings obtained safely by qualified personnel. It does not prescribe live probing, test motion or parameter writes.
1. Read the complete alarm and its Target column
The overall AL.20 name is Encoder normal communication error 1: an error occurred in communication between an encoder and the servo amplifier. The detail names for 20.5 and 20.6 are Transmission data error 1 and Transmission data error 2. Neither name proves that a particular conductor is broken or that the encoder itself has failed.
On page 42, cause (1) for both details concerns incorrect wiring when an A/B/Z-phase differential-output linear encoder is used. Its Target column lists only [A], [B] and [GF]. Causes (2)–(7) also list [WB] and [RJ010] and refer to the AL.20.1 check method. Consequently, the whole 20.5/20.6 alarm must not be described as exclusive to linear encoders, and the phase-wiring branch must not be generalized to every rotary motor serial encoder.
Printed page 4 defines [A] as the listed MR-J4-A models, including applicable DU and 03A6 models; [B] as the listed MR-J4-B and DU models; [GF] as the listed MR-J4-GF models; [WB] as the MR-J4W multi-axis models; and [RJ010] as MR-J4-_B-RJ010/MR-J4-_B4-RJ010. These are troubleshooting groups, not a guarantee that every model in a group supports a particular encoder interface. Record the complete amplifier model, suffix, software version, encoder model and actual interface combination, then check that model's function and wiring manuals.
2. What distinguishes the two specific branches?
| Detail | Cause (1) check on page 42 | Prerequisite |
|---|---|---|
| 20.5 | Disconnection or short circuit of A/B-phase pulse signals PA, PAR, PB and PBR | An A/B/Z-phase differential-output linear encoder in an applicable [A]/[B]/[GF] configuration |
| 20.6 | Disconnection or short circuit of Z-phase pulse signals PZ/PZR | The same prerequisite |
These are signal names, not universal connector pin numbers. Identify connectors, pins and conversion units from the actual configuration. For a confirmed disconnection or short circuit, this branch specifies encoder cable repair. If neither is found, it directs the reader to cause (2); it does not declare the entire feedback system healthy. Inferring a faulty homing program from the name 20.6 also exceeds the evidence in this table.
3. What follows if the phase wiring shows no fault?
The remaining page 42 causes include a faulty cable, an encoder cable external conductor not connected to the connector ground plate, incorrect parameters for a differential-output linear encoder, amplifier malfunction, encoder malfunction and environmental influences. Their check method refers to AL.20.1 on page 41. The external-conductor connection is part of signal-cable integrity; knowing that the cabinet is grounded does not replace checking the relevant connector.
The communication-method check on page 41 references PC22 for [A] and PC04 for [B]/[WB]/[RJ010]/[GF]. It separately lists PF40 for a parallel drive system. These references help locate the correct manual entries; they are not instructions to change parameters. Compare saved settings with the original design and applicable edition only. Do not copy a similarly named parameter, default or another machine's backup into this machine. Component substitution and repeatability tests require a separate diagnostic plan by qualified service personnel; one alarm does not justify jumping straight to motor replacement.
4. Obtain evidence safely
- First stop and prevent unexpected movement under the machine's safety procedure. Manual page 6 requires SON off and power interrupted when an alarm occurs. Preserving records must not delay a safe stop.
- Collect the complete detail number, axis, time and existing power-up, operating or cable-maintenance records. Establish the feedback device type without deliberately recreating the alarm.
- Qualified personnel must isolate the equipment and secure the load before wiring or inspection. Safety page 2 requires at least 15 minutes after power-off, or 20 minutes for a converter unit, the charge lamp off, and verification with a suitable tester that P+ to N− voltage (L+ to L− for a converter) is safe. Time elapsed or a dark lamp alone is insufficient.
- Record cable, connection and environmental findings under the confirmed branch. Do not hot-plug, forcibly move the motor, bypass protection or command motion to observe an alarm.
- Only after the cause is removed and safety confirmed should authorized personnel restore operation under the machine procedure. Safety page 3 requires the run signal off before alarm reset to prevent sudden restart. An alarm disappearing after reset does not itself prove the root cause is gone.
5. Hypothetical example: one detail, two paths
Suppose machine A has a verified A/B/Z-phase differential-output linear encoder and a recorded 20.6. Qualified personnel would prioritize the PZ/PZR branch using the actual wiring diagram. If no disconnection or short is found, they continue with cable connections, the external conductor and the remaining listed factors. Suppose machine B has a rotary motor serial encoder and also records 20.6. The table does not justify inventing PZ/PZR test points on that interface; use its actual interface documentation and the general communication-check path. This is a hypothetical manual-reading example, not a field case or measured result.
Sources and limits
MELSERVO-J4 Servo Amplifier Instruction Manual (Troubleshooting), SH(NA)-030109ENG-T, November 2025: printed page 4 (models and Target definitions); §1.1, page 6 (alarm precautions); §1.4, pages 41–42 (AL.20.1, 20.5 and 20.6); Safety Precautions, pages 2–3. PDF viewer page numbering differs from printed pagination. No universal pinout, recommended parameter value or unverified repair result is provided.