How to Fix ABB ACS880 Fault 5681 PU Communication Guide

Staring at Fault 5681 on your ABB ACS880? This PU Communication failure means the control unit can't talk to the power unit. Learn how to fix it here.

 

How to Fix ABB ACS880 Fault 5681: PU Communication Failure

If you are looking at your ABB ACS880 and it's displaying Fault 5681, you are dealing with a classic "communication breakdown." In the ACS880 manual, this is defined as a PU Communication error.

In simple terms, the Control Unit (the brain where your wires connect) has lost its connection with the Power Unit (the hardware that actually turns the motor). When these two stops talking, the drive trips immediately for safety. Let's look at why this happens and how to fix it without immediately buying expensive spare parts.

Common Causes of the 5681 Fault

Because the ACS880 is used in everything from small pumps to massive cranes, the cause can vary based on the "Frame Size" of your drive:

• Loose Fiber Optic Cables: In larger ACS880 frames (R6 and up), the brain and muscles talk via fiber optics. If these are loose or dirty, you get a 5681.
• Poor Ribbon Cable Connection: In smaller wall-mounted units, a flat ribbon cable or direct pins connect the boards. Vibration can shake these loose.
• Faulty Power Unit Power Supply: If the Power Unit loses its internal 24V supply, it "goes dark," and the Control Unit can't find it.
• Electromagnetic Interference (EMI): High electrical noise can disrupt the data flowing between the units.

Step-by-Step Troubleshooting Guide

1. The "Fiber Optic" Check (For Large Drives)

If your ACS880 has a BCU (Black Control Unit) separate from the power modules, check the fiber optic links:

• Look for the small red and black cables. Are they plugged in all the way? You should hear a distinct "click."
• The Light Test: Unplug one end safely. Do you see a red light glowing through the cable? If not, the cable is broken or the transmitter has failed.
• Cleanliness: Use a can of compressed air to blow out any dust from the fiber ports. Even a tiny speck of dust can block the signal.

2. Reseat the Control Unit (For Small Drives)

If your control unit is mounted directly on the front of the drive:

• Power down the drive completely and wait for the lights to go off.
• Carefully remove the control unit and check the pins on the back.
• Ensure there are no bent pins or signs of "arcing" (black burn marks). Snap it back into place firmly.

3. Verify Parameter 95.04 (Control Board Supply)

This is a "secret" fix that often saves the day.

• Check Parameter 95.04. This tells the drive if the control board is powered by an external 24V or the drive's internal power.
• If this setting is wrong, the drive may try to talk to a power unit that hasn't "woken up" yet, triggering the 5681 fault.

4. Check for Blown Internal Fuses

If the 5681 fault is accompanied by other communication errors, check the small glass fuses on the ZCU or BCU control board. If the board isn't getting power, it can't talk to the power unit.

Can You Reset Fault 5681?

You can try to press Reset, but if the communication link is physically broken, the fault will return instantly. Unlike an overload fault, 5681 requires the hardware connection to be restored before it will clear.

Summary

The ABB ACS880 5681 fault is all about the internal link. Before you assume a board is dead, perform the "Tug Test" on your fiber optics and check your 95.04 parameters. Most of the time, it's just a loose connection or a settings mismatch after a part swap.