1000

Illegal Command or Format

Allen-Bradley · Logix and SLC PLC

What does 1000 mean?

This error indicates that the local processor received a command or message that is not recognized or is improperly formatted. This prevents the processor from executing the requested operation, leading to an inability to control or communicate with the target device.

Common Causes

  • Command syntax error in the local processor's message instruction.
  • Unsupported command type sent to the local processor.
  • Incorrect message structure preventing proper interpretation by the processor.

Repair Steps & Checklist

Click steps to track your progress.

  1. 1

    Verify the PLC program logic and data types related to the communication interface for correctly formatted commands or expected responses according to the target device's protocol.

  2. 2

    Check all communication parameters (e.g., protocol settings, IP addresses, baud rates, data types, message structures) configured in the PLC and any connected communication devices for consistency and correctness.

  3. 3

    Inspect all communication cables, connectors, and network infrastructure components for physical damage, loose connections, or signs of interference that could corrupt data packets.

  4. 4

    Verify firmware versions of the PLC, its communication modules, and any connected intelligent devices for compatibility, ensuring they meet the required specifications for the implemented communication protocol.

  5. 5

    Examine the configuration and operational status of the device attempting to communicate with the PLC for any faults or incorrect settings that might cause it to send illegal or malformed commands.

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Verified technical data. Last updated: March 2026

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Source: Allen-Bradley Logix and SLC PLC