25 Aug
2014
25 Aug
'14
8:22 a.m.
Interesting, but I'm not sure what a strobe (signal) is.
I'm surprised that no one on the list has turned up with an S-100 SRAM card
for me! Those things are actually all over eBay, must be 20-30 listed right
now, in the $59-300 range. The top-end of the prices are for a genuine MITS
4K DRAM board - the short-lived, ill-fated one that about lost them the
business.
On Sun, Aug 24, 2014 at 11:48 PM, Eric Smith <spacewar at gmail.com> wrote:
On Sun, Aug 24, 2014 at 5:07 PM, Eric Smith
<spacewar at gmail.com> wrote:
By which Tony means "can sink a **MUCH**
larger current than it can
source".
If you look at the spec for the TI SN7404, the
difference is a factor of
40:
max -0.4 mA source (high), vs. max 16 mA sink
(low). Generally speaking,
0.4mA is nowhere near enough to be usable for an LED, so driving the low
side of the LED was the only practical approach with normal TTL.
This is also why many parts offered "open collector" outputs, where the
high
side transistor of the totem pole was omitted, allowing wire-OR (negative
logic)
or wire-AND (positive logic) with a pullup resistor, but there weren't TTL
chips
that only had the high side drive, since that was too wimpy to be very
useful.
Even with CMOS, where the equivalent is "open drain", there usually aren't
high-side only outputs, though with the symmetric drive of modern CMOS
that would work just as well.
That's also why strobes are almost always negative logic (active low).
Eric