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Data transfer between a computer and an external device is another main function of I/O. There are two main types data transfer - parallel and serial. Parallel data transfers multiple bits simultaneously, thereby achieving higher information transfer rates. Serial data transfer only allows information to be transferred one bit at a time. Serial data transfer requires fewer hardware pins.

Circuits for parallel data transfer are similar to those circuits described for sensory input and control output. There are not many variations of parallel data transfer formats.

There are a lot of variations of serial data transfer in terms of protocol and physical signal specifications. This subject is too broad to be discussed in this short course so detailed description of various serial data communication will be deferred to another course. Only a very brief introduction of serial data transfer is given here.