24 Aug
2014
24 Aug
'14
9:48 p.m.
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