SPI and I2C protocols are suitable for communicating with multiple slave subsystems, as they are designed for multi-device communication over the same bus, unlike Serial RS232 and USB.
If communication to multiple slave subsystems is required, two communication protocols that would be suitable are the Serial Peripheral Interface (SPI) and the Inter-Integrated Circuit (I2C). Unlike Serial RS232 and Universal Serial Bus (USB), both SPI and I2C are designed specifically for short-distance, serial communication in embedded systems and can easily facilitate communication with multiple devices over the same bus. SPI uses a master-slave architecture with separate input and output lines, allowing for full-duplex communication. Conversely, I2C uses a two-wire bus, which is more limited in speed compared to SPI but can address multiple slave devices with only two wires by using different addresses for each slave.