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Transmission of data on a serial line accomplished byrapidly changing a single output bit, in software, at theappropriate times. The technique is a simple loop with eightOUT and SHIFT instruction pairs for each byte. Input is moreinteresting. And full-duplex (doing input and output at thesame time) is one way to separate the real hackers from thewannabees.Bit bang was used on certain early models of Primecomputers, presumably when UARTs were too expensive, and onarchaic Zilog Z80 micros with a Zilog PIO but no SIO. Inan interesting instance of the cycle of reincarnation, thistechnique is now (1991) coming back into use on some RISCarchitectures because it consumes such an infinitesimal partof the processor that it actually makes sense not to have aUART.