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From: henkp@nikhefk.UUCP (Henk Peek)
Newsgroups: sci.electronics
Subject: Re: TTL Questions
Message-ID: <192@nikhefk.UUCP>
Date: Sat, 22-Aug-87 19:46:36 EDT
Article-I.D.: nikhefk.192
Posted: Sat Aug 22 19:46:36 1987
Date-Received: Sun, 23-Aug-87 21:41:57 EDT
References: <7105@alice.UUCP> <764@sol.ARPA> <128@umich.UUCP> <20036@ucbvax.BERKELEY.EDU> <18007@amdcad.AMD.COM>
Reply-To: henkp@nikhefk.UUCP (Henk Peek)
Organization: Nikhef-K, Amsterdam (the Netherlands).
Lines: 21
Keywords: TTL, pullups

In article <18007@amdcad.AMD.COM> rpw3@amdcad.UUCP (Rob Warnock) writes:
|In article <20036@ucbvax.BERKELEY.EDU>, after pointing out quite
|correctly that in 7400-series (not 74LS') one must *NOT* tie inputs
|to the +5 rail, Max Hauser writes:
|+---------------
|| I should add that one never, of course, uses a resistor in series
|| with inputs tied to ground, for current-sinking logic such as TTL
|| families (of all kinds), since the major input current flows in the
|| "low" state, causing a voltage drop that can cloud the logic "0";
|| and reverse junction breakdown is not an issue in that case.
|+---------------
|Well... there are times when a resistor to ground makes sense, too.
|Notice that any resistor value less than "V_max_0/(I_max_0 * #of_inputs)"
|will be fine, as it's no worse than a true gate output would be. For a
|7400 TTL single "standard load" input, that's 0.4/.016 = 25 ohms or less.
                                                         ########	

The (low) input curent of a standard single load 7400 TTL input is 1.6 mA.
The maxium serie R for a single load standard input is 0.4/0.0016 = 250 ohms.

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