Re: OT - TTL Circuit: Pullup vs Supply

Hmmmm, sounds like there are arguments both ways... go figure. I like
the rationale of the 1K resistor tho that Zonn and Rodger gave. If I can
route the traces, I will go with the resistor, if the routing gods don't
agree with me, then I will tie them to +5V then. :-) Maybe I will be
swank and use a 470 Ohm. :-)

John Robertson wrote:
> At 3:12 AM -0500 9/8/06, Matt J. McCullar wrote:
>> > I could swear I recall reading something in the "applications notes"
>>> section of one of my old TTL books about a pull-up (or pull-down)
>>> resistor
>>> providing improved noise immunity, which might be important for
>>> control
>>> pins (such as reset* / preset* pins on flip-flops or shift registers)
>> whose
>>> function isn't needed in the application in question...
>>>
>>> ...but I'll be damned if I can *find* the book in question! The books
>>> I have handy don't say anything about this... so I've either
>> lost/misplaced
>>> the book that did, or I'm completely misremembering having ever
>>> read it in
>>> the first place. :)
>>> --------------------------------------------------------------------------
>>>
>> ---
>>
>> I'll try to look in Don Lancaster's _TTL Handbook_ (or whatever he
>> calls it)
>> for this info. My copy is at work, not at home.
>>
>
> Page 12/13 of Don Lancaster's bible "TTL COOKBOOK" states that inputs
> should be either tied directly to Vcc or to a "logically similar
> input". he goes on to state that the military practice of tying unused
> inputs to Vcc via a 1K resistor is "usually not justified by the
> benefits and causes more problems than it solves".
>
> I recommend that any tech without a copy of this and CMOS COOKBOOK
> should get them into their library ASAP! eBay often sells these for
> under $20US, and Amazon.com and other booksellers are often much less
> for used books.
>
> John :-#)#
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Received on Fri Sep 8 17:15:35 2006