Bit Banger(s) we all have known.

[Tony Duell]

> Well (since you asked) the original serial interface for the HP 2116
> computer (around 1966 or so) was a serial bit-banging interface.  You
> needed to assemble the byte from the ASR-33 (or 35, take your pick)
> teletype a bit at a time.  They later had the bit assembly/disassembly
> register (this WAS pre UART chips) that did al the work.  Unfortunately
> the "parallel" serial interface was only half duplex, as it used the
> same register for both assembly and disassembly.  The "serial" (bit-
> banging)  serial interface IIRC did not have this restriction.

COme to think of it, that's very similar to the serial interfaces for the 
98x0 calculators.

The hardware one only had a single shift register for both Tx and Rx. I 
think there was a status line brought out (at RS232 levels) on one of the 
handshake pins that indicated to the peripheral when the calcultor was 
expecting input. It's a half-duplex device.

The bit-banger had no common circuitry between the input and output, and 
could be used in full-duplex mode. It was only supported on the 9830 
(BASIC-language) machine, but I can't see a hardware reason why there 
couldn't have been firmware written for it on the other machines in the 
range.

-tony