> 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.

When Centronics designed the interface which would become the standard, not
only was the pull-up speed not the limiting factor, but communications PERIOD
was not a limiting factor.  Simply put--on the printers of the time, a data
transfer rate of 1,000 bytes/second was more than adequate (even a typical
dot matrix printer outputting graphics could only print about 500 columns of
dots per second).  Actually, being limitted to 1,000 bytes/second as some
computers were did pose something of a limitation since many printers of the
time could not simultaneously print and receive data; even a small (2K or so)
buffer between the computer and printer could improve graphics throughput
quite noticeably.

[slight editing of my >> quoted text to improve line lengths]
>>The Toshiba's pullup on the strobe is, at least from my perception, even
>>stiffer than a 220 ohm resistor; as for changing paradigm, there aren't
>>any definitive electrical specs for the old parallel port and designing
>>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...

Certainly there were specifications for the Centronics interface, but since
sending a thousand or so bytes of data down a cable isn't particularly hard
many parameters either were not specified rigidly or else were often imple-
mented contrary to the specification.

> 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.

My point was that the Toshiba laptop has substituted for the older paradigm
(put a fairly stiff pullup on the strobe wire) a new paradigm (put a really
incredibly solid active pullup on the strobe wire) which was a departure
from the old one but which may have been desired to improve transfer speeds
(220 ohms is fine for sending 1,000 bytes/second or even 25,000 bytes/second
but may not be good enough when sending 1,000,000 bytes/second).  My point
was simply that it would have been less of a departure from the old paradigm
of they'd made it so that the pullup was mega-stiff only briefly when the
line made a transition and was otherwise only moderately stiff.

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

Heh... though some of the equipment then was pretty impressive.  For example,
there are times when I have a lot of source code to print out and I'm waiting
for the HP LJ/4p's to print (4ppm) when I've wished I had one of those nice
old fashioned chain printers next-door (I don't think I'd want one in _MY_
office...)