The RS-232 standard is an asynchronous serial protocol primarily used to establish serial communications between DTE and DCE devices.
For example between a MCU and a PC running an application.
The SPI protocol is a synchronous serial protocol primarily used to establish serial communications between a MCU and a peripheral device.
For example, between an 8051 variant and Real Time Clock (RTC) with a SPI interface.
They are normally never required to be implemented for the same interface.
You might use an RS-232 connection to transmit information to your PC, which originated from a peripheral device attached to your MCU via an SPI interface.
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