The work presents the modular design of an embedded system for smart lockers to optimize the control and supervision of the data flow using 8-bit microcontrollers. For this work, serial communication was implemented to link a central computer with the master-slave system. The master-slave system executes the control and response frames for the client and the administrator. This research develops a multitasking embedded system for the Atmega 2560 microcontroller. It also implements RTOS (Real Time Operating System). The inputs and outputs are programmed in C, and their signals are transformed into communication commands sent by the RS485 protocol to the central computer so that, through the RJ45 cable, the modular design of the lockers is connected to interact by frames. This design proposes the implementation of RTOS with low-cost microcontrollers to create a robust and functional modular system to achieve a plug-and-play connection of up to 32 serial lockers, allowing access to information and interaction with the lockers without altering the flow of communication.