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DPRG: Logic Families

Subject: DPRG: Logic Families
Date: Tue Aug 24 19:45:50 CDT 1999

)I was with Kip when we discovered just how he got bitten. I had the
)benefit of working actively in design when each of the different logic
)families up through 74HC came on line.

I was there for the 8xx/9xx DTL and for TTL and LSTTL, and for 4xxx and
74C. 74C was *very* static sensitive (as were the 4xxx). They were also
S L O W. That, of course, like every undesirable quality of any new
product, was a "feature". It was called "Controlled Rise and Fall

)Thanks for taking time to do the chart -- it will be a welcome addition
)to the DPRG reference library some of us are compiling. I only have one
)quibble -- I believe 35 NHz is too high for 74XX -- I believe that is the
)rating for 74SXX. 

Quite welcome. I didn't intend to mean that *every* chip in the 74xx
line was good to 35MHz. I seem to recall the 7490 or at least one of
the counters (7474?) being rated at that speed. Typical propagation
delay was considered 10ns. If four gate delays were present in a 7474
(master/slave JK F/F, two gate delays each for master and slave), then
that would be 40ns, equivalent to 25MHz, I guess.

I recall 74S being good to 50MHz, give or take. But again, that may not
be the whole line. And I may be remembering how fast an oscillator could
be built from a couple of gates, rather than rated speeds.

If you really want to add it to the library, you probably should check
out the actual values. I do recall the currents for the TTL, because I
had to use them frequently. I also recall that the heavy-duty 4xxx
(like the 4049 which we used all the time) were equivalent more or less
to the LSTTL in drive capability, which was about 2 TTL and definitely
10 LSTTL loads. So I did the computation to get the sink current for
LSTTL necessary for a logic low. The regular 4xxx weren't good even for
2 TTL loads, though they were usually considered ok for a few LSTTL
loads. When the "B" series 4xxx came out, they were a little better for
drive, but not much.

I still have troubles handling the HC/HCT, even though in my mind I
tell myself "These are not 4xxx chips, they won't blow just because you
look at them wrong!" I still feel queasy using them at home. But that's
about all you can get these days!

OTOH, I never blew a CMOS chip or transistor in my life. I've known
many who did, but I never did. So either I'm lucky, overcautious, or
just cautious enough. Who can tell?

I stopped my electronics about 10 years ago, and then started up again
about 2 years ago. I still can't get over CMOS being reasonably robust,
and always take static precautions. I have a "poor man's" safety
surface: Al foil on the dining room table, taped down. :)

I always keep one or more parts of my body in contact with that surface
when dealing with HC or HCT.

)Aside from a fairly good memory, I remember a project I had to analyze the
)HARM command launch computer. The design prototype used only 74XX; they
)upgraded initially to 74SXX so they could run at 24 MHz instead of 8
)MHz of the prototype. The rest of the story is interesting, but too long
)for here and now.

I have several friends who worked on HARM and a few more who worked on
TOAD. You know

	David Tierce
	Rick Cook
	Robert (Rock) Landry
	John Telford


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