The Master-Slave is a model for a communication protocol or system architecture where one device, the "master," controls one or more other devices, the "slaves." In this model, the master device initiates all communication transactions. It sends commands or requests to the slaves and is the only device that can start a dialogue on the communication bus. The slave devices listen for commands addressed to them by the master and respond accordingly. They do not initiate communication themselves and only speak when spoken to.
This hierarchical model is very common in digital electronics and control systems because it provides a simple and orderly way to manage communication with multiple peripherals. Examples of protocols that use a master/slave architecture include I2C, SPI, and Modbus. For instance, in an RS-485 multi-drop system, a single master computer might send commands to multiple slave power supplies connected to the same bus, addressing each one by its unique slave ID.
It is important to note that due to the negative historical connotations of the term "master/slave," the technology industry is actively moving away from this language. Recommended alternative terms are now widely used to describe this same architecture, including "primary/secondary," "leader/follower," "controller/device," "primary/replica," and "primary/standby." These newer terms describe the technical relationship just as effectively without using socially charged language.