Using optocoupler for galvanic isolation of RS-232 Bus introduced in this app note from Vishay. Link here (PDF)
The RS-232 bus is a serial, single-ended bus system with a data transfer rate up to 115.2 kBd. The application areas are computer to computer or computer to peripheral devices communication, like a printer, mouse, and so on.
In case of electrical noise, coupling effects, or level shifting requirements, an optocoupler is a perfect solution for isolating two devices. Due to galvanic isolation, an optocoupler helps to break up ground loops and reduce the electrical noise due to optical light transmission across an isolation barrier. It can also restore the signal logic level and further be used for signal level transforming between different voltage domains, e.g. from 5 V to 3.3 V.