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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 transfer 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 among serial data transfer protocols 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 to serial data transfer is given here.