5 Jun
2014
5 Jun
'14
7:45 p.m.
On 06/05/2014 05:57 PM, Eric Smith wrote:
Still needed some 40mA drivers, didn't it? The
industry-standard floppy
drives still used 150 ohm termination at that time. (I still don't really
understand how the later PC drives work reliably with 1K termination using
100-110 ohm impedance ribbon cable and no real control over the source
impedance.)
But they do, just fine. Probably because the signals aren't really that
fast. At any rate, I can find absolutely no mention of the output drive
capability of the 1770/1772/1773--not even in the application notes.
Very strange.
Outputs to drives are low-true, however, so the usual 7438 wouldn't be
appropriate without an inverter beforehand. I can't recall, but I don't
believe that the ST used any output buffering--but it's been too long to
be certain.
By using their own drive design, with only the cabling
designed into their
products, and not having to support long cables or stubs, Apple avoided
needing high-current drivers. That might even have saved more money in
reduced PCB area than the actual components.
A possibility, but at the expense of compatibility with anything but
another Mac.
--Chuck