> On Sun, 22 Dec 1996, John Payson wrote:
> > Do you (or does anyone) have any idea whether that wierdness is only a
> > problem with Toshiba laptops, or if it is more widespread?  I have no
> > real qualm with the /Strobe wire not being open-collector (if it's open
> > collector it will pose a bottleneck when trying to send data quickly to
>
> This proves not to be the case.  That interface was designed in the early
> days of TTL, and is designed to have a pull-up that is by modern standards
> an awfully small value - I recall 220 ohms or 330 ohms as being common for
> plain-old-TTL pullups.  And in fact, the Centronics interface was spec'd
> with setup and strobe (minimum) pulse width on the order of a couple
> microseconds not for the sake of the logic gates but to allow for a
> reasonably long cable.

220 ohms pullup... with 4.4 volts across it (pulling down to 0.6volts) that's
20 mils.  Plausible I guess, but even so a PIC should be able to sink that
with no problem.  On the Toshiba, though, the pullup on that thing was REAL
STIFF--even a PIC output couldn't pull it very far (now THAT's stiff).

> > [btw, even for the strobe wire, they should have done something analagous
> > to the 87C51 pullup design: make it so that when the pin has a low->high
> > transition it will be pulled up hard, briefly, and so that it's pulled up
> > weakly the rest of the time.]
>
> That would have been interesting, but I'm afraid you're forgetting how old
> the design is.  They used an earlier design paradigm's "hard" pull-up
> rather than one that hadn't been invented yet.  :-)

The Toshiba's pullup on the strobe wire is, at least from my perception, even
stiffer than a 220 ohm resistor; as for changing paradigm, there really aren't
any definitive electrical specifications for the old parallel port and design-
ing a circuit so that a rising edge on "strobe" would switch a 50 ohm pullup
for 2us followed by leaving on a 4.7K (or even 330ohm) pullup would have been
quite in line with the specifications.

Oh well...