Moin,
On Sun, 11 Nov 2007 18:45:28 -0600
Terry Hancock <[EMAIL PROTECTED]> wrote:
> > The incompatibilities are in the voltage levels for high/low only.
> > CMOS uses level relative to Vcc, while TTL has fixed (and incompatible
> > positioned) levels. Though nothing that could not be fixed with a simple
> > pull up resistor.
>
> I was under the impression that a TTL input would suck down a CMOS
> output due to the power consumption, damaging the CMOS chip. TTL is
> relatively low impedence, so it flows current whenever it is on (hence
> the high power dissipation -- TTL chips get *hot*), while the CMOS is
> high impedence, so it doesn't flow much current except during switching.
> (Or so has been my limited understanding -- this fits with my experience
> of discrete bipolar and FET transistors, though).
Uhm.. Good question. Now that you mention this, i'm not
sure anymore whether it was really only the high level of TTL
outputs being a bit low. It's been decades (literaly) since
i last read about TTL 74s and i didnt use much of them even then.
Can anyone of the old guys here confirm this? I don't have any
databooks from that time left (only a big book with the description
of the logic of most 74s).
> But I've long since lost track what was what and what the technology is
> like today. I just know there's a lot of alphabet soup that gets stuck
> into the labels, but they're conceptually the same chips.
I lost track of most of everything i used to know :-(
These days, I usualy stick with datasheets of the devices
i currently use when i have to use them.
> > But these days a lot of
> > the chips are single gates, which have a pseudo standard
> > (ie most use the same pinout, but not all).
>
> But why would they use 7400 labels for those at all? That's just evil.
Because if i tell you it's a single gate 7414 then you know
w/o thinking that it's a schmitt trigger inverter. Try to fit
"schmitt trigger inverter" on a 2x3mm^2 SMD package :)
(Mostly these packages do not even carry the "74" anymore, but
just "AC14")
> > (side note: most of the 74s with higher pin counts and
> > more complex functionality were replaced by CPLDs and FPGAs
> > and thus died out)
>
> Anyway, all the datasheets I've pulled up by searching for 7400 series
> numbers (so far) have been compatible with the pinouts in my (C)1980
> "The Master IC Cookbook", so I think we're pretty safe.
The 74s are standard and don't change. Anyone selling a 74xx with
different pinout than the standard is just evil (and wont sell much).
The only exception are said single gate devices.
Attila Kinali
--
Praised are the Fountains of Shelieth, the silver harp of the waters,
But blest in my name forever this stream that stanched my thirst!
-- Deed of Morred
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