11/23 clock issue

[Holm Tiffe]

Eric Smith wrote:

> On Fri, Feb 6, 2015 at 12:44 AM, Holm Tiffe <holm at freibergnet.de> wrote:
> > You don't ned no pullup for +5. All open TTL inputs are reading High w/o
> > any pullup.
> 
> That's partially correct for "real" TTL, but not for an CMOS "TTL",
> such as 74HC, HCT, AC, ACT, FC, FCT, BCT, LVC, etc.  In other words,
> partially correct for "plain" 74xxx, 74L, 74H, 74S, 74LS, 74AS, 74ALS,
> 74F, but not many of the other "TTL" families which are actually CMOS.
> 
> The reason I say "partially" is that an open TTL input has very poor
> noise immunity. From a DC perspective it is pulled high, but in
> practice noise pulses can easily be coupled into it.  It has always
> been considered poor practices to leave TTL inputs open.  And
> *extremely* poor practice for CMOS.

Whow. All this is related to an FOX Crytall Oscillator driving a 74S240 on
a M8186 Board?

Holm

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