1. A method comprising: determining, by processing circuitry of a remote terminal unit (RTU), that a portable storage medium (PSM) is coupled to the processing circuitry; and in response to determining that the PSM stores function code in a specified folder that the processing circuitry is configured to access, performing a specified function corresponding to the function code by executing the function code, wherein the RTU comprises a slot configured to physically receive the PSM and electrically couple the PSM to the processing circuitry, and an on-chip memory mounted on a same chip as the processing circuitry.