9 Feb
2015
9 Feb
'15
12:11 a.m.
Eric Smith wrote:
On Fri, Feb 6, 2015 at 12:44 AM, Holm Tiffe <holm
at freibergnet.de> wrote:
Whow. All this is related to an FOX Crytall Oscillator driving a 74S240 on
a M8186 Board?
Holm
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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.