How to Reset an Allen-Bradley PLC (CompactLogix, ControlLogix, MicroLogix)

When your Allen-Bradley PLC locks up or throws a persistent fault, time is of the essence. Whether you're dealing with a CompactLogix, ControlLogix, or MicroLogix system, knowing how to reset it correctly can save hours of downtime. In this quick guide, we’ll walk through the safest and most effective reset methods—plus key troubleshooting tips—so you can get your system back online fast.

When Should You Reset a PLC?

Resetting is typically needed when your PLC shows any of the following:

• Unresponsive behavior after a power outage or update
• Status lights (OK, FLT) are blinking or solid red
• Major or minor fault errors in RSLogix or Studio 5000
• Startup hangs during boot sequence

Resetting clears temporary system faults, but it won't fix hardware issues or corrupted programs. If the problem returns after a reset, further troubleshooting may be needed.

Method 1: Power Cycle Reset

This is the simplest method and should be tried first.

1. Turn off power to the PLC completely (remove all voltage).
2. Wait at least 30 seconds to allow full discharge.
3. Restore power and observe the status indicators.

Tip: For MicroLogix units like the Allen-Bradley 1766-L32BXB, also check for external I/O faults or blown fuses after reboot.

Method 2: Software Fault Reset (Using RSLogix or Studio 5000)

Use this method if your PLC is faulted but still communicating via Ethernet or serial connection.

1. Connect your PC to the PLC using RSLogix 500, RSLogix 5000, or Studio 5000 (based on model).
2. Open the project file or go online.
3. Navigate to the Controller Fault Handler or Status pane.
4. Right-click the fault and choose “Clear Major Fault.”
5. Switch the controller mode back to “Run.”

Works for: CompactLogix (e.g., 1769-L32E), ControlLogix (e.g., 1756-L71), MicroLogix 1400

Method 3: Key Switch Reset (ControlLogix & Some CompactLogix Models)

If your controller has a physical key switch, you can cycle modes to reset it.

1. Turn the key to “Program” mode.
2. Wait 5–10 seconds.
3. Turn the key to “Run” or “Remote Run.”

This is a soft reset that clears certain faults without affecting the loaded program.

Method 4: Memory Reset (Full Factory Default)

Use this only as a last resort. It wipes all user programs and configuration data.

1. Power off the controller.
2. Remove the battery or short the capacitor terminal (if applicable).
3. Hold the “Reset” or “Clear Memory” button (if present) while powering back on.
4. Wait for LEDs to flash or the controller to return to default state.

Warning: You’ll need to reload your ladder logic or function block program from a backup.

Common Fault Indicators and What They Mean

Status Light	Color/Behavior	Meaning
OK	Solid Green	Normal operation
OK	Flashing Red	Non-recoverable fault
FLT	Solid Red	Major system error (check logs)
BAT	Flashing	Battery low or missing (for models with memory backup)

Quick Troubleshooting Tips

• Try a fresh USB or Ethernet cable if RSLogix can’t detect your PLC.
• Check the controller’s IP address and ensure your PC is on the same subnet.
• Remove recently added modules if the issue began after hardware changes.
• Look up specific fault codes in Rockwell’s manuals for targeted fixes.
  
  

  Looking for an Allen-Bradley PLC Alternative?

  If your existing Allen-Bradley PLC is obsolete, unavailable, or too expensive to replace, there are several industrial-grade alternatives that offer similar functionality, programming structure, and reliability.

  While IAC does not source Allen-Bradley PLCs, we carry compatible alternatives that can perform the same core control tasks with a minimal learning curve for technicians already familiar with AB systems.

  Your Allen-Bradley PLC	IAC Alternative	Why It Works
  ControlLogix	Siemens S7-1500	Similar rack-based architecture and enterprise-level performance
  CompactLogix	Siemens S7-1200	Compact modular design with EtherNet/IP communication support
  CompactLogix	Mitsubishi FX5U	High-speed processing, built-in Ethernet, and flexible expansion
  MicroLogix 1100/1400	Omron CP1E / NX Series	Comparable I/O capacity with simple programming structure
  MicroLogix	Schneider Modicon M221	Compact footprint with modern communication capabilities

  All of these platforms support IEC 61131-3 Ladder Logic programming, making the transition easier for controls engineers and maintenance teams already experienced with Allen-Bradley environments.

  For facilities facing discontinued hardware, long lead times, or urgent downtime situations, these PLC families provide reliable replacement paths without requiring a complete controls redesign.

 

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