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The mystery of the illogical logic gate

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Salio
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re: The mystery of the illogical logic gate
Salio   10/1/2010 3:46:23 PM
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I can’t forget the undergraduate days when I used to be in the lab trying to figure why the outputs of TTL gates are not what they should be. I have run into the problem you described many more times than I wanted to. I had approached the problem from a digital system point of view before I checked the power and the ground pins. Had I checked the power and ground pins first to make sure that they are working the way they should be, I would have saved a lot of time. In retrospect, it was a good experience to go through when something doesn’t work. When I took my senior level chip design class, the first or second lab we had to design 64 kilobytes of RAM circuit with TTL gates. Of course, it didn’t work in the first go through but it eventually did after long sleepless nights.

ylshih
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re: The mystery of the illogical logic gate
ylshih   10/1/2010 5:39:40 AM
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There are a whole host of problems that will arise from assuming the following: Ground is Ground, V+ is V+, Reset never glitches! :-)

Patk0317
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re: The mystery of the illogical logic gate
Patk0317   9/28/2010 4:18:05 PM
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Warren, Sounds like that was an interesting project. o you know if the Galois Field Arithmetic is still in use?

bdf
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re: The mystery of the illogical logic gate
bdf   9/27/2010 11:44:29 PM
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In the past, I had to deal with a similar situation. The PCB I was working with was behaving really weird, but not at all times... The long story short, you must always make sure that the reset pins of all components on the PCB are, first of all, connected and, second of all, connected to the right places. Otherwise it's like opening Pandora's box... :)

agk
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re: The mystery of the illogical logic gate
agk   9/27/2010 8:29:50 AM
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The first thing always i test in my boards newly assembled are the supply voltages to all active components. Secondly measure or monitor the current consumed and whether it matches with the design value nearly. Then i go for dynamic testing of the boards. This way i had been saving lot of time and energy.

wilber_xbox
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re: The mystery of the illogical logic gate
wilber_xbox   9/25/2010 10:38:10 PM
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the kind of story to which we all can relate. Engineering is atleast based on logic and in the end logical thinking solves the problem and not thinking outside the box.

alkey3
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re: The mystery of the illogical logic gate
alkey3   9/25/2010 9:25:08 PM
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Hey Warren, Solving your first problem like that is exciting! I still get that thrill when I figure out some design or implementation error. In fact I will often crank up one of my favorite rock anthems and celebrate! Yup, I am still a geek at heart!

WKetel
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re: The mystery of the illogical logic gate
WKetel   9/25/2010 1:30:47 AM
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Missing and non-soldered connections on an IC can indeed be quite challenging, that is certain. That is why we would have an inspector, who did not nee to know electronics, just pin 1 and transistor connections. IN that organization, the inspector was very valuable.

Jon.Jonsen
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re: The mystery of the illogical logic gate
Jon.Jonsen   9/24/2010 8:25:52 PM
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I didn't think of a NAND. I did look up the schematic of an AND. The AND in question had no power supplied. So it could not function normally. I can believe that leakage current from high inputs could turn on the output pull down giving low out. I don't see why all four inputs high do not result in low out if less than four do.

araasch
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re: The mystery of the illogical logic gate
araasch   9/24/2010 7:56:32 PM
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Jon: With 7400 series TTL logic an open on the input is the same as a high. (It is not recommended to depend on this fact in production circuits, but it is done) If all inputs were driven high the AND gate actually functioned and output a high. You may have been thinking he was describing a NAND gate where all high inputs gives you a low output.

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