Encoder Communication Error
E.740 indicates loss of communication between the servo drive and the connected rotary encoder (absolute or incremental). This fault affects closed-loop position feedback and causes the drive to disable servo control immediately, as position verification cannot be established or maintained during operation.
Error Code
E.740
Severity
Fault
Equipment
Servo Drive
Manufacturer
Mitsubishi
Drive LED displays red error indicator with E.740 code on display panel
Motor loses torque and becomes free-spinning despite control signal input
Servo alarm relay trips, disabling connected downstream equipment
No encoder pulse detection shown in drive diagnostics menu (0 counts/revolution)
Communication timeout message appears in parameter tree after 3-5 seconds of power-up
Axis position error accumulates rapidly, triggering position deviation protection
Encoder cable disconnected, pinched, or damaged at drive or motor connector (most common)
Encoder connector pins corroded or loose from vibration in harsh industrial environment
Encoder power supply voltage sagging below 4.5V due to undersized cable gauge or load sharing
Incremental encoder count-per-revolution mismatch in drive parameter settings vs actual encoder
Encoder signal line shielding broken, causing EMI interference from nearby VFD or welding equipment
Internal encoder failure (Hall sensor bridge failure, optical disk damage, or bearing seizure)
Power off drive completely and verify 24VDC encoder supply voltage at terminal block (should read 24V ยฑ5%); if low, check cable gauge and source capacity
Disconnect encoder cable from drive rear connector and visually inspect all 4-5 pins for corrosion, bent contacts, or debris; clean with compressed air and contact cleaner if needed
Measure DC resistance of encoder signal wires A/B/Z phases: should read 50-150ฮฉ between phases; open circuit (infinite ฮฉ) indicates broken wire or internal failure
Reconnect encoder cable firmly until click is heard; gently rotate motor shaft by hand and confirm encoder LED on drive flashes with each revolution (visual encoder activity test)
Access drive parameter menu and verify encoder resolution setting matches mechanical encoder (e.g., 2048 PPR for standard servo encoders); incorrect setting causes false E.740
If resistance test fails or LED shows no activity, replace encoder cable with new shielded twisted-pair rated for servo duty; route away from AC power lines by >20cm minimum
If cable replacement fails, run drive self-diagnostics test (hold Setup + Enter for 5 seconds); if encoder count still reads zero, encoder unit requires replacement
After repair, perform zero-position calibration routine and run 10-revolution test cycle to confirm stable position feedback before returning to production
Safety First: Always isolate power and follow LOTO (Lockout/Tagout) procedures before working on electrical components. If unsure, contact a certified repair technician.
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