Re: Apple II Parallel Cards

["Alex Freed" <alexf@mirrow.com>]

"Michael Black" <et472@FreeNet.Carleton.CA> wrote in message
news:ed6s8i$pu9$1@theodyn.ncf.ca...

> Remember, the "IBM PC" parallel port was not two-way for a long time.
> When people needed both ways (for non-printer use), they'd get by
> with the input lines that were there (not a full byte's worth) in
> the form of handshaking lines.  Or, one article (I think it was in
> Micro Cornucopia), showed soldering some ICs onto a parallel card,
> to add 8 bits of input.

I didn't see the article but here is some info I do know. There were many
clones, but the original IBM PC
printer port had readable input from the 8 data lines, but no way to disable
the driver. So if your device had stronger
drivers than the PC, you *could* read back 8 bits. Interestingly enough the
driver chip was tri-state (74LS244, I think) and there
was an unused bit in the configuration register. So a simple mod used just
an exacto knife and a wire to make
the port bi-directional.

I worked for Adobe Systems at the time. In the "systems" devision i.e.
making PRINTERS rather than
Photoshop, etc.
And out PostScript printers did talk back. So some bi-dir functionality was
needed. After a long series of
experiments it was decided to use ECP or EPP mode if available and 4 bit
back channel if not.

Programs like Laplink used this mode too. Much faster than serial.


-Alex.