How to Troubleshoot ABB ACS550 Alarm A2021 Start Enable 1 Missing?

Getting the ABB ACS550 Alarm A2021? This "Start Enable 1 Missing" warning means your drive is missing its safety/run permission signal. Learn how to fix digital inputs and parameter 1608.

 

The ABB ACS550 is designed with several layers of "permission" signals. Even if you press the "Start" button, the drive will refuse to run if it doesn't receive a specific safety or operational signal. If your display is flashing Alarm A2021 (Start Enable 1 Missing), the drive is effectively waiting for permission to operate.

This is a protective feature. If your machine's safety gate is open, or if your PLC has decided it's not safe to run, the drive will sit in a "Standby" mode and display this alarm until the "Start Enable" condition is satisfied.

What Triggers Alarm A2021?

The ACS550 checks its input terminals (or communication data) to see if the "Start Enable 1" signal is high (24V DC). If that signal is low or missing, the drive will not output power to the motor.

• Open Safety Circuit: An emergency stop button or safety interlock connected to a digital input is open.
• Incorrect Parameter Configuration: Parameter 1608 is pointing to a digital input that isn't receiving 24V.
• PLC Communication Failure: If you are controlling the drive via Fieldbus, the PLC has not sent the "Start Enable" bit.
• Loose Wiring: A wire has come loose from the digital input terminal on the drive's control board.

Step-by-Step Troubleshooting Guide

1. Check Parameter 1608

The first step is identifying where the drive is looking for this signal.

• Navigate to Menu > Parameters > Group 16 System Controls.
• Check Parameter 1608 (START ENABLE 1).
• If it is set to a digital input (e.g., DI1, DI2, etc.), the drive requires 24V DC to be physically present on that specific input terminal.
• If it is set to COMM, the drive is expecting this signal via your Fieldbus (Modbus, Profibus, etc.).

2. Test the Digital Input (Hardware)

If Parameter 1608 is assigned to a Digital Input:

• Use a multimeter to measure the voltage on that specific DI terminal (referenced to the COM terminal).
• If you measure 0V, you must trace the wiring back to the source (E-stop button, relay, or switch).
• If the switch is closed but you still have 0V, there may be a break in the wire or the external switch itself has failed.

3. Check for Interlocks

Often, "Start Enable 1" is used for safety interlocks, such as:

• A flow switch on a pump.
• A damper end-switch on a fan.
• A machine cover or enclosure door interlock.

Verify that all mechanical interlocks in your system are in the "closed/ready" position.

4. Communication Settings (If using PLC)

If you are using Fieldbus control:

• Verify that your PLC code is actively sending the "Start Enable" command.
• Check your communication registers to ensure the drive is successfully receiving the data packet. If the PLC is in "Program" mode, it may have stopped sending these enable bits.

Summary Checklist

Troubleshooting Step	What to look for
Parameter 1608	Confirm which input the drive is watching.
Voltage Check	Confirm 24V DC is present at the assigned terminal.
Field Devices	Check E-stops, limit switches, and safety relays.

Pro-Tip

If you are trying to test the drive and keep hitting the A2021 alarm, you can temporarily set Parameter 1608 to NOT SEL. Warning: Only do this for temporary diagnostic purposes. Once set to NOT SEL, the drive will ignore the missing signal and allow you to test the motor rotation. Always change it back to the original setting when commissioning is complete!