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From: wtm@neoucom.UUCP
Newsgroups: sci.electronics
Subject: Re: TTL Questions
Message-ID: <647@neoucom.UUCP>
Date: Sun, 9-Aug-87 22:25:37 EDT
Article-I.D.: neoucom.647
Posted: Sun Aug  9 22:25:37 1987
Date-Received: Tue, 11-Aug-87 01:36:57 EDT
References: <7105@alice.UUCP> <764@sol.ARPA> <128@umich.UUCP>
Organization: Northeastern Ohio Universities College of Medicine
Lines: 21
Keywords: TTL, pullups
Summary: It is good form to use a pull-up resistor.


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 are often spikes on Vcc that are
considerably greater than the normal nominal operating voltage.
Exposing the inputs to voltages greater than the votage to the
power supply pin of the chip can stress the input, possibly
shortening the life of the chip.  The pull-up resistor limits the
current when a spike occurs.  A 74704 with the input to one of
the inverters tied low is handy for terminating inputs that need to
float high.-- Although tieing low to a poor ground can endanger the
chip too.  In home brew projects, terminating unused inputs to +5
volts is probably alright, but I wouldn't do it in a piece of
equipment I'm planning to sell to somebody else.  One resistor per
every 10 to 15 unused inputs really isn't asking much.-- If you
have a lot of unused inputs, you probably haven't designed your
equipemnt very well.  If the design is going on a PC boad, the cost
of adding the resistors and traces isn't excessive when amortized
over a production run.

  --Bill