A system includes a remote terminal unit (RTU) and a portable storage medium (PSM), such as a secure digital (SD) card. The RTU includes processing circuitry. The RTU includes a portable storage interface configured to physically connect to the PSM and electrically couple the PSM to the processing circuitry. The RTU includes on-chip memory. The processing circuitry is configured to determine that the PSM is coupled to the processing circuitry and access a specified folder of the PSM. The processing circuitry is configured to: in response to determining that the PSM stores function code in the specified folder, perform a specified function corresponding to the function code by executing the function code.