Re: Motherboard chip descriptions?

[Don Bruder <dakidd@sonic.net>]

In article <sH%cd.416976$Fg5.10642@attbi_s53>,
 "Ernest" <leucoplast@seanet.NOSPAMS.com> wrote:

> Does anyone know of a book or website that describes what each of the
> different ICs (74LS* chips) on the Apple II and clone motherboards do? I'm
> going to be starting a "compare and contrast" project between the Apple II+
> and my different clone motherboards, which often have some interesting
> differences. If I can figure out what each of the different chips are for,
> then I might be able to better understand why chips were added or removed on
> different boards. That's my hope anyway.

7400-series chips are "commodity" logic chips. Depending on the number 
on it, it might be an "OR" gate, a "NAND" gate, or any of several steps 
of complexity, all the way up to a complete subsystem such as a one-chip 
stepper-motor driver or mux/demux.

Punch any one of them that you're interested in into google, adding a 
space, and the magic incantation "+datasheet" after the numbers. Don't 
give up if the first try gets you nothing - If you've got, for instance, 
a chip labeled 74HCT123N, try it first exactly as-labeled. If that 
returns no hits, try it as 74HCT123. If that gets you nowhere, try it as 
74LS123, and keep on trying until you've found a match - the letters in 
the middle are different specs for the same chip - an "LS" chip is built 
around the "Low-power Schottky" technology, the "HCT"s perform just a 
touch differently (electrically speaking - identical as far as logic 
operation and pinout are concerned) than their LS counterparts, which 
makes them useful for certain "trick" ways of getting a job done (An 
oscillator built out of a nor gate, two caps, two resistors, and a 
crystal, ferinstance - Because of electrical differences, the "LS" chip 
won't oscillate at all, but the "HCT" version will give you a gorgeous 
square-wave at the crystal's rated speed) and so on. The trailing letter 
(usually only one, but sometimes two letters) usually indicates package 
type and/or temperature range - Perhaps a "C" on a ceramic-case version 
of the chip, with a "P" on the plastic version, and similar. 

Once you've found the datasheet for the chip, you've got everything 
you'll ever need to know about it (and probably some extra that you 
didn't even know COULD be known!)

On a related note: If you find you need substitutes for any reason, and 
you're having a hard time finding one - Or perhaps you encounter one on 
one of those clone boards - let's say you need a replacement 74LS323 - 
You can *USUALLY* change the "74" to "54", pay a little extra, and get 
yourself the military/hostile environment version of the chip - So for 
the '323 you need, you can probably find it as a 54LS323, and it will be 
pin-compatible with the 74LS323 version. Likewise, if you encounter a 
54xxx chip, it's just the military/hostile environment version of the 
74xxx. Note that the 74/54 crossover dosn't necessarily work well for 
the "tricky" stuff like the NOR-gate oscillator I mentioned above - For 
situations where you're using the chip purely as a logic gate/module, 
you should never see a problem. Start playing with "weird" stuff like 
the oscillator example, and you may very well find that what works 
perfectly with a 74xxx chip in the socket either doesn't do anything at 
all, or worse - but unlikely - "lets all the smoke leak out" when a 
54xxx chip is put in its place.

-- 
Don Bruder - dakidd@sonic.net - New Email policy in effect as of Feb. 21, 2004.
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