Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!husc6!hao!oddjob!gargoyle!ihnp4!kitty!larry
From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: TTL Questions
Message-ID: <1925@kitty.UUCP>
Date: Mon, 10-Aug-87 19:16:16 EDT
Article-I.D.: kitty.1925
Posted: Mon Aug 10 19:16:16 1987
Date-Received: Wed, 12-Aug-87 01:07:42 EDT
References: <7105@alice.UUCP> <764@sol.ARPA> <128@umich.UUCP> <647@neoucom.UUCP>
Organization: Recognition Research Corp., Clarence, NY
Lines: 27
Keywords: TTL, pullups
Summary: Another reason to use pull-up resistors...

In article <647@neoucom.UUCP>, wtm@neoucom.UUCP (Bill Mayhew) writes:
> Manufacturers recommend tieing unused inputs to logic level high
> via a pull-up mainly for the safety of the chip in electrically
> noisy environments.

	There is another reason why I use pull-up resistors for unused logic
level high inputs: it makes it easier to implement post-production design
revisions (since the resistor can be easily removed if necessary, or the
pin simply tied elsewhere in the circuitry if the pin is to be pulled down
to logic low).
	While it is not possible to use such a trick when pins are to be
tied to logic low level, if I anticipate a potential future logic change,
I have been known to pull a pin low by employing an unused negative logic
gate or inverter (whose input is pulled high using a resistor).
	I call the above design technique "ECO-logic". :-)

> One resistor per
> every 10 to 15 unused inputs really isn't asking much.

	I would never do this; either use individual resistors, or tie
the inputs to Vcc if you must - but using a common resistor across a
number of logic elements is an invitation to Transient Trouble.

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
<>  UUCP:  {allegra|ames|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry
<>  VOICE: 716/688-1231       {hplabs|ihnp4|mtune|seismo|utzoo}!/
<>  FAX:   716/741-9635 {G1,G2,G3 modes}   "Have you hugged your cat today?"