Source : Free On-Line Dictionary of Computing

bit bang
     
        Transmission of data on a {serial line} accomplished by
        rapidly changing a single output bit, in software, at the
        appropriate times.  The technique is a simple loop with eight
        OUT and SHIFT instruction pairs for each byte.  Input is more
        interesting.  And {full-duplex} (doing input and output at the
        same time) is one way to separate the real hackers from the
        {wannabee}s.
     
        Bit bang was used on certain early models of {Prime}
        computers, presumably when {UART}s were too expensive, and on
        archaic {Zilog Z80} micros with a {Zilog} PIO but no SIO.  In
        an interesting instance of the {cycle of reincarnation}, this
        technique is now (1991) coming back into use on some {RISC}
        architectures because it consumes such an infinitesimal part
        of the processor that it actually makes sense not to have a
        {UART}.
     
        [{Jargon File}]