Modern PLCs can be programmed over ethernet. Some such as Allen Bradley Control Logix  have a physical key switch on the front which allows the PLC to be put in run&#x2F;program&#x2F;remote mode so it could be locked to run mode which would prevent remote changing of the mode and not allow the program to be modified.<p>I&#x27;ve never worked with Siemens PLCs apart from a demo from the distributor and the schneider PLCs I work with don&#x27;t have the physical run&#x2F;program switch.