JargonFile/entries/bit bang.txt

bit bang

n. Transmission of data on a serial line, when accomplished by rapidly
tweaking 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 UARTs were too expensive, and on archaic Z80 micros with a
Zilog PIO but no SIO. In an interesting instance of the cycle of
reincarnation , this technique returned to use in the early 1990s on some
RISC architectures because it consumes such an infinitesimal part of the
processor that it actually makes sense not to have a UART. Compare cycle of
reincarnation. Nowadays it's used to describe I2C, a serial protocol for
monitoring motherboard hardware.