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From: john@hpcvlo.HP.COM (John Eaton)
Newsgroups: sci.electronics
Subject: Re: TTL Questions
Message-ID: <1090002@hpcvlo.HP.COM>
Date: Tue, 11-Aug-87 11:54:41 EDT
Article-I.D.: hpcvlo.1090002
Posted: Tue Aug 11 11:54:41 1987
Date-Received: Sat, 15-Aug-87 01:32:35 EDT
References: <1395@crash.CTS.COM>
Organization: Hewlett-Packard Co., Corvallis, OR, USA
Lines: 18

<<<<
< In the recent discussion on TTL there seem to be numerous
< recommendations to use a series resistor when connecting an unused
< input to +5 volts.  This is an unnecessary waste of a resistor, the PC
< board space to mount it, and the foil trace to connect it.  Can't we
< finally give pullup resistors a decent burial?  Unused inputs should
< be connected DIRECTLY to the appropriate (+5 Volt or Gnd) supply rail!
----------

  I suspect that the real reason that everybody does this is that they all
suspect that the circuit may not work as designed. Having some unused inputs
not tied to a rail is very handy if you have to cut traces and add little 
yellow wires to a board to get it into production.


John Eaton
!hplabs!hp-pcd!john