On Sun, 22 Dec 1996, John Payson wrote:

> 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).

Oops - I wasn't talking about the Toshiba, about the quirks of which I am
quite happy not to have to know <grin>; rather I was addressing the
issue of switching speed with a pullup.  For a moderate length of
twisted-pair wiring, which was what the old Centronics spec called for,
the use of pull-up instead of some active current source was not even
close to being the limiting factor.

> 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.

There may not be any electrical specifications for the IBM parallel port,
but that's because they were (nominally, at least) designing to the
relatively ancient Centronics interface.  I have no idea if there was ever
a formal standard sanctioned by a national standards body, but there
certainly were published specs for that interface.  Granted that no one
has likely felt the need to publish them in the last fifteen or twenty
years...

And yeah, it was an earlier paradigm.  Think for a moment how much
additional circuitry it takes to add that dynamic pullup when you're
building it out of SSI TTL gates and discrete components.  Back then, if
you needed X mA of pullup you simply designed that in as a static load, as
they did.  It would be unfair to say that they didn't care how much power
it took; rather it was just that this wasn't an unusual expenditure of
power for the time.

Not that I would want to go back to those design rules... pocket
calculators that could heat good-sized rooms and such.  :-(  :-)