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Imagine this: It is 2:00 AM on a production line. A critical Siemens S7-300 PLC has failed. Replacement hardware is on hand, but when a technician tries to upload the original program from the CPU to a new engineering workstation, they are met with the dreaded pop-up: "The unit is protected by a password."
When a password is lost, the CPU will show as "Access denied" in STEP 7 (Classic) or TIA Portal. Standard upload attempts fail.