Hello... need some help here... are the LS/HC/and HCT series IC's compatible? Ie., is a 74LS139 compatible with the 74HCT139? Any info/help would be much appreiciated.
Thanks.
Lyos Gemini Norezel

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Hi Everyone,

You can learn the difference by going to the "logic" portion of the major
chip manufacturers web-sites (that still sell logic chips). There, look through the application notes for an overview of different logic families. They will have it!

Try

Texas Instruments
Fairchild
ST
Toshiba

among others.

Cheers,

Paul

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At salower speeds, all are ocmpatible to a point. The HC series being a CMOS does NOT have the larger fanout of the LS and S series. For the real wierd families, I have absolutely no clue. But in general, if you are driving very little load at slower speed, they should all work. As a rule of tyhumb, I go by:

LS: general purpose
HC: low current CMOS, TTL compatible
S high speed

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"My biggest dream in life? Building black plywood Habitrails"

Hi Everyone,

There is a lot to this subject so I again suggest that you read the APP notes from the manufacturers.

For example:

0) HC is slower than LS.
1) HC is not TTL compatible (HCT is though, but still slower).

Cheers,

Paul

Thanks for the info/links... I appreiciate the help.
Lyos Gemini Norezel

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Mundus Vult Decipi et Decipiatur

> The HC series being a CMOS does NOT have the larger fanout of the LS and S series.

For the 74xx139 you specifically asked about, my 1988 National Semiconductor data book specifically says of the 74HC139 (not even HCT139): "The decoder's outputs can drive 10 low power Schottky TTL equivalent loads" (their exact wording). The fanout given for LS is 20. So although the HC-to-LS fanout is less, it's still probably more than enough for what most people on this forum want to do.

> 0) HC is slower than LS.

I've heard this many times; but according to my National Semiconductor and Fairchild databooks, HC is generally a little faster-- not in every single case, but usually. They make a point of that in the ap. notes as well.

> 1) HC is not TTL compatible (HCT is though, but still slower).

It's not 100% compatible, but they can be used with each other under certain conditions. The 65 family's inputs are all geared toward TTL voltage thresholds; but when everything in the circuit is some kind of MOS (NMOS, CMOS, etc.), HC works perfectly fine because all the 1's will be driven right near 5V and the 0's right near ground anyway, so the "turnover" point becomes irrelevant. For many years I've used HC almost exclusively with the 65 family, with no problems. Putting in the occasional LS part until I could get the HC I originally wanted has not been a problem functionally, although the LS part takes a lot more power supply current. Now I mostly use 74AC or ACT.

74ACT is both compatible and much faster than either HC or LS.

Again, see the many ap. notes with links in the 6522 v. WDC 65c22 forum subject.

Hi Everyone,

Its all in the details.....

Here are two data sheets from Ti.

http://focus.ti.com/lit/ds/symlink/sn74ls541.pdf

http://focus.ti.com/lit/ds/symlink/sn74hc541.pdf

74LS540
======
Propagation delay typical = 9 nsec
Propagation delay max = 15 nsec

74HC540
======
Propagation delay typical = 9 nsec
Propagation delay max = 19-22 nsec

Remember some chips fall into the "max" bin when they are shipped.
Not all are "typical".

Also, LS cannot drive HC without pull-ups! HC can drive LS with no problem. Maybe this is what Garth was thinking.


Cheers,

Paul

"Also, LS cannot drive HC without pull-ups!"

Not strictly true. There are two problems.

One is that the voltages are different.

A TTL input is spec'd to see anything over ~2V (1.8V actually, IIRC) as a logic high,
and the outputs are spec'd to pull up to a minimum ~2.4V (actually 2.45V IIRC)
CMOS is usually very symetrical and has a threshold of close to 1/2 VCC, ie 2.5V,
so the TTL spec. doesn't gaurentee what CMOS "wants"
for a high, voltage-wise.
Never the less the TTL high output voltage is usually in the 3-4V range
(I think 3.5V usually given as typical) and it's seldom a problem. (In my experience)

The other problem is that TTL doesn't source much current, so
even if it can attain a high enough voltage, it may not do it very fast
(it may not be able to charge the CMOS input capacitance quickly).
And that voltage may not be very high compared to what the CMOS
would "like". Both of which can slowdown the CMOS the TTL is driving.

Worse, the slow transitions can cause problems like not clocking or erratic behavior
because of a minimum clock speed spec. (not fast enough
for some AC coupled internal CMOS thing or the CMOS oscillating as the input lingers near the threshold).

And it'll be more susceptible to noise (especially as it passes though the threshold).

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What's an additional VIA among friends, anyhow?

Hi Everyone,

Good discussion. Thanks for the update.

Cheers,

Paul

I often use the 74ALS family as a modern replacement of the 74LS. It's faster than the 74HCT (4ns prop. delay vs. 9ns), and still draws less power than 74LS, but now as little as the HCT.

André

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_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Here's a good site that lists chip family comparisons. One family that the site doesn't mention is the 74ABT series, which is often used in bus driver chips, such as the 74ABT541. The 74ABT family is very fast (single digit nanosecond performance) and has high fanout. Their outputs, however, at TTL, not CMOS.

The families I recommend for new designs are 74AC and 74HC. The former is very fast, but also may give rise to noise and ringing problems if physical construction isn't right. 74HC is more tolerant of sloppy construction.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?