ABB ACS580 Fault 5681 PU Communication Error
Getting the 5681 PU communication fault on your ABB ACS580 drive? Learn how to check internal connections and configure Parameter 95.04 to restore communication.
If you are working with an ABB ACS580 general-purpose drive and the control panel suddenly displays Fault 5681 (PU communication), your drive has hit a major internal roadblock. In simple terms, the "Brain" of the drive is no longer talking to the "Muscles."
In the ACS580 architecture, the Control Unit (CU) handles all the logic and parameters, while the Power Unit (PU) handles the actual electricity flow to the motor. Fault 5681 triggers when there is a communication breakdown between these two components. If they can’t talk, the drive cannot safely operate the motor. Here is a human-made guide to help you find the "broken link" and fix it.
What Triggers Fault 5681?
The 5681 error is almost always a physical connectivity issue or a power supply configuration mismatch. It rarely points to a settings error within your motor data, but rather a fundamental failure in the drive's internal "nervous system."
Step 1: Check the Physical Connection
In 80% of cases, Fault 5681 is caused by a loose ribbon cable or a connector that has wiggled free due to machine vibration or improper installation during a recent repair.
- Inspect the Ribbon Cable: Open the drive cover and locate the flat ribbon cable (or the direct-mount pins) that connects the control unit to the power module.
- Reseat the Connector: Gently unplug the cable and inspect the pins for dust, corrosion, or bending. Re-insert it firmly until you feel it "click" or seat properly.
- Check for Damage: Ensure the cable isn't pinched or frayed. A single broken strand in that communication wire will trigger a 5681 fault instantly.
Step 2: Verify Parameter 95.04 (Control Board Supply)
This is the "hidden" culprit. The ACS580 needs to know how its control board is getting power. If the setting doesn't match the physical reality, the communication handshake will fail.
Navigate to Menu > Parameters > Complete List > Group 95 (HW Configuration) and check 95.04:
- Internal 24V: This is the default. The control board gets its power directly from the drive's internal power supply (the PU). If this is selected but the PU has a failing internal power supply, communication will drop.
- External 24V: Use this if you have a separate 24V DC power supply wired to the control unit terminals.
The Conflict: If you are using an external supply but 95.04 is set to "Internal," or vice versa, the drive may experience synchronization errors, leading to the 5681 trip. Ensure this parameter matches your physical wiring layout.
Step 3: Check for Electromagnetic Interference (EMI)
If the fault is intermittent—meaning it only happens when the motor starts or when a nearby machine cycles on—you likely have a noise problem.
- Ensure the control unit is properly grounded to the drive frame.
- Check that the communication cable between the CU and PU is not routed too close to high-power motor leads.
Summary Checklist for Fault 5681
| Checkpoint | Action Required |
|---|---|
| Ribbon Cable | Unplug and re-seat. Ensure pins are clean. |
| Parameter 95.04 | Ensure it correctly identifies the 24V source (Internal vs. External). |
| Internal LEDs | Check for a green "Power" LED on both the CU and the PU. |
| Environment | Check for loose mounting screws that might cause excessive vibration. |
Conclusion: Is the Hardware Failed?
If you have re-seated the cables and verified that Parameter 95.04 is correct, but Fault 5681 remains, you may have a permanent hardware failure. This usually means either the Control Unit (CCU) or the Power Module interface has suffered a component failure.
At this stage, swapping the Control Unit with a known-good one (if available) is the best way to isolate which board has failed. If the fault persists with a new Control Unit, the Power Unit's internal communication board is the culprit and will require professional repair.
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