How to Fix ABB ACS560 Warning A3A1 STO Feedback Mismatch

Seeing A3A1 on your ABB ACS560? This Saf Torque Off (STO) feedback mismatch warning means your safety channels are out of sync. Learn how to fix the wiring here.

 

Solving ABB ACS560 Warning A3A1: STO Feedback Mismatch

If your ABB ACS560 is flashing the A3A1 warning, the drive has detected a disagreement in its own safety system. This specific code stands for SAF (Safe Torque Off) Feedback Mismatch.

In simple terms, the drive has two separate channels for its safety circuit. For the motor to run, both channels must be "On." If one channel thinks the machine should be stopped and the other thinks it should be running, the drive gets confused and triggers the A3A1 alarm. It's a "handshake" that failed.

Common Causes of the A3A1 Warning

Because the A3A1 code is usually a wiring or timing issue, look for these four common culprits:

• Broken or Loose Wire: A single wire on the 4-pin XSTO terminal has come loose due to vibration.
• Faulty Safety Relay: The external safety relay (like a Pilz or Schneider unit) has one "sticky" contact that is opening slower than the other.
• Missing Jumper: If you aren't using an external safety relay, one of the two factory-installed yellow jumpers is missing or loose.
• Contact Bounce: High vibration is causing the safety contacts to "chatter," momentarily desyncing the channels.

Step-by-Step Troubleshooting for A3A1

1. The "Tug Test" on the XSTO Terminal

Locate the XSTO terminal block (usually a small, distinct connector on the drive control unit).

• There are four pins: 1 (+24V), 2 (GND), 3 (STO1), and 4 (STO2).
• Give each wire a tiny tug. If one slides out, you’ve found your A3A1 culprit. Ensure all connections are tight.

2. Use a Multimeter to Check Voltage

With the safety relay "Closed" (meaning the machine should be ready to run), measure the DC voltage at the drive:

• Measure between Terminal 2 (GND) and Terminal 3 (STO1). You should see 24V DC.
• Measure between Terminal 2 (GND) and Terminal 4 (STO2). You should also see 24V DC.
• If one is 24V and the other is 0V, the mismatch is in your external safety circuit (E-stop, gate switch, or relay).

3. Check Safety Relay Timing

If the A3A1 warning only happens occasionally right when you hit the E-stop or reset the safety circuit, your safety relay might be failing. If the two contacts inside the relay don't move at the exact same millisecond, the ACS560 will notice the tiny delay and throw the A3A1 code.

4. Verify Parameter 31.22 (STO Indication)

While this won't "fix" a broken wire, check Parameter 31.22. This parameter determines if the drive shows a Warning (Alarm) or a Fault when the STO circuit is triggered. If it's set incorrectly, it can make troubleshooting more confusing by hiding the real status of the safety inputs.

How to Clear the A3A1 Warning

1. Fix the loose wire or replace the faulty safety relay contact.
2. Ensure both STO1 and STO2 terminals have a solid 24V DC signal.
3. Once both channels are synchronized, the A3A1 warning will usually disappear from the screen automatically. Unlike a fault, you often don't need to press "Reset."

Summary

The ABB ACS560 A3A1 warning is an essential safety guardrail. It tells you that your safety system is no longer "redundant"—meaning if one channel failed, you wouldn't be fully protected. Usually, it's just a loose wire on the XSTO connector. Tighten your terminals, check your voltages, and you'll be back in production in no time.