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
wannabees.
Bit bang was used on certain early models of Prime computers, presumably when
UARTs 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]
