21 Aug
2005
21 Aug
'05
10:57 a.m.
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