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Putting the switch to MRES (Memory Reset) clears the user program and the password. However, if the MMC card contains a password-protected program, the CPU will reload it from the MMC on startup. You must remove the MMC first.
If you have a locked PLC and without a backup, wiping the PLC is out of the question. Because the S7-300 stores everything natively on its Micro Memory Card (MMC), the password hash exists in raw binary format on the card. Required Hardware & Software unlock s7300 plc password work
have been used to retrieve password data from MMC images in older systems. S7CanOpener:
Unlock S7-300 PLC Password Work: Methods, Risks, and Legal Alternatives This public link is valid for 7 days
Siemens implemented a mechanism. When a password is set on an S7-300 CPU, it is stored in the internal flash memory of the PLC. Crucially, this password is not stored in plain text in the project file on your hard drive; it is verified against the hardware itself.
Several third-party tools market themselves as "S7-300 Password Unlockers." These tools typically exploit legacy vulnerabilities in the MPI or Profibus communication protocols utilized by older S7-300 firmware versions (V2.x and early V3.x). How They Work: Can’t copy the link right now
This is the most common and straightforward situation for a factory engineer who simply needs to reuse a controller for a new task. Since there is no program to preserve, performing a hard reset is the recommended path.