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From: dave@onfcanim.UUCP (Dave Martindale)
Newsgroups: sci.electronics
Subject: Re: TTL Questions
Message-ID: <15370@onfcanim.UUCP>
Date: Mon, 17-Aug-87 10:46:33 EDT
Article-I.D.: onfcanim.15370
Posted: Mon Aug 17 10:46:33 1987
Date-Received: Thu, 20-Aug-87 02:02:38 EDT
References: <7105@alice.UUCP> <764@sol.ARPA> <128@umich.UUCP>
Reply-To: dave@onfcanim.UUCP (Dave Martindale)
Organization: National Film Board / Office national du film, Montreal
Lines: 26
Keywords: TTL, pullups

In article <128@umich.UUCP> don@umich.UUCP (Don Winsor) writes:
>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!

Well, if you read the data book for 7400 TTL carefully, you'll see that
it is *not* recommended to tie inputs to Vcc, and that a pullup resistor
should be used.  The reason is that TTL input transistors may break down
at 5.5V, and there is the possibility that the power supply voltage may
go above that value momentarily during turnon or turnoff.  The resistor
limits the current to a value that will not destroy the transistor.

Now, 74LS parts (and others) use diodes instead of a multi-emitter
transistor, and can withstand 7V, so the resistor is not necessary.

It is safe to omit the resistor only if you know that the input in question
can handle the higher voltage.  A general rule of "no resistors" is not
safe.

>
>If all else fails, read the manufacturers data book.

Good advice indeed.