Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!seismo!rutgers!clyde!cbosgd!cwruecmp!hal!oxtrap!b-tech!metavax!umich!don
From: don@umich.UUCP (Don Winsor)
Newsgroups: sci.electronics
Subject: Re: TTL Questions
Message-ID: <128@umich.UUCP>
Date: Thu, 30-Jul-87 13:35:48 EDT
Article-I.D.: umich.128
Posted: Thu Jul 30 13:35:48 1987
Date-Received: Sun, 9-Aug-87 21:40:41 EDT
References: <7105@alice.UUCP> <764@sol.ARPA>
Organization: EECS, University of Michigan
Lines: 41
Keywords: TTL, pullups
Summary: Connect unused TTL high inputs directly to +5

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!

A couple of quotes from the net:
] ... it's generally good practice to use a small
] resistor when you attach any input to a rail.
) ... One easy way of dealing with spare inputs on N/AND, N/OR
) gates is to tie them in parallel to another input.  As opposed
) to pulling them high with a resistor to +5V.  Provided you
) stay within the load capacity of the driving gate.
Neither of these is correct for LS, ALS, or FAST TTL.

From Motorola Schottky TTL Data, Series D, Third Printing, 1983:

    UNUSED INPUTS.  For best noise immunity and switching speed,
    unused TTL inputs should not be left floating, but should be
    held between 2.4 V and the absolute maximum input voltage.

    Two possible ways of handling unused inputs are:

    1.  Connect unused input to Vcc.  LS, ALS, and FAST
        TTL inputs have a breakdown voltage > 7.0 V and
        require, therefore, no series resistor.

    2.  Connect the unused input to the output
        of an unused gate that is forced HIGH.

    CAUTION:  Do not connect an unused LS, ALS or FAST input to
    another input of the same NAND or AND function.  This method,
    recommended for normal TTL, increases the input coupling
    capacitance and thus reduces the ac noise immunity.

If all else fails, read the manufacturers data book.

Don Winsor         Ann Arbor, Michigan
University of Michigan EECS Department
... {ihnp4,msudoc,itivax,umix,symplex,arbortext,m-net,aat}!umich!don