Password Unlock | Koyo Plc
Sometimes, the error message "Password Rejected" appears even when you are entering the correct code. This is likely a communication hardware issue.
| Symptom | Likely Cause | Solution | | :--- | :--- | :--- | | Immediate "Fail" after 1 attempt | Wrong baud rate or parity | Set DirectSOFT to "Autodetect" | | Fail after 3 seconds | Poor ground loop | Use a short, shielded serial cable | | "No response from PLC" | USB adapter driver mismatch | Install FTDI drivers, avoid CH340 | | Password works, but no online edits | The "Master Lock" switch is on | Check physical keyswitch on CPU | koyo plc password unlock
Title:
"Recovering Access to Legacy KOYO PLCs: Password Mechanisms, Vulnerabilities, and Authorized Recovery Methods" Sometimes, the password only prevents online edits
Abstract:
This paper reviews the password protection system implemented in legacy KOYO programmable logic controllers (PLCs), particularly the Direct Logic series (e.g., DL205, DL305, DL405). It discusses common reasons for password loss (e.g., forgotten credentials, personnel turnover, lost documentation) and explores both official recovery procedures and known technical weaknesses. Emphasis is placed on ethical considerations, legal boundaries, and safe practices for authorized personnel. avoid CH340 |
| Password works
Sometimes, the password only prevents online edits. Try the "Upload" function. Many machine builders password the "Write" function but leave "Read" open for troubleshooting. If you can upload the logic, you can strip the password offline.
Some Koyo PLC models have a built-in password recovery feature. To use this method:
Attempting to bypass PLC passwords without explicit authorization can be illegal and unethical. Unauthorized access may violate computer misuse statutes, contract terms, and industry regulations. Even with good intentions, bypassing protections can expose operators and bystanders to safety hazards. Always obtain written permission and follow organizational policies.