Apple 1 C8 chip - 7450 or 7451 ?

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Apple 1 C8 chip - 7450 or 7451 ?

Hello,

As I showed in my previous topic, I am finishing building two Apple 1 replicas.https://www.applefritter.com/content/my-version-apple-1-replica

Everything works as it should. Most of the TTL chips are from my inventory.

While browsing the photos of the original Apple from 1976, I noticed that the C8 position is the 7451 and not the 7450 (as in the original schematic and PCB description). This interested me, and I know the differences between the 7450 and the 7451. I also know that the 7451 or 74S51 is not the same as the 74LS51 (2 or 3 NAND inputs).

Its fine.

BUT, does anyone know why there are these discrepancies between schematic / PCB and finished products?

BTW, 7450 were widely available in my country (UCY7450 from CEMI - Polish semiconductor manufacturer during the Cold War and Communism), but the 7451 were not available at the time.

In my resource I found 7451 ITT with datecode 7229 (CDIP) which I used in one of the replicas and it works fine.

Regards,

Mateusz / SQ9PXB

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 Erratum.I know that probably

 

Erratum.

I know that probably does not interest anyone, but order should be kept. UCY7451 were produced in Poland during the communist era. Found them in my old stocks... They are also in the CEMI catalog.

 

Regarding my question, no one knows how it was historically with the 7450 - 7451 in Apple 1?

 

 

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7450,7451 and 74LS51 in Apple-1

In post #2, SQ9PXB asked:

 

"Regarding my question, no one knows how it was historically with the 7450 - 7451 in Apple 1 ?"

 

Uncle Bernie says:

 

I seem to dimly remember there has been a discussion about this topic in this forum several times in different threads and IIRC, the 7451 was spotted in original Apple-1. But I'm too lazy to look, the photos are on the web. All I know for sure is that both types work in the Apple-1. Heck, even the 74LS51 works just fine in the Apple-1.

 

- Uncle Bernie

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Just checked, on my photos

Just checked, on my photos from the #38 "Nixdorf Museum" (NTI version) you can see that it's a 7451 there.

For whatever reason there is a 7451 at all, you can simply leave the two expansion inputs X and /X of a 7450 open. Maybe it was the cheaper alternative back then. (...?)

Uncle Bernie, are you sure that even the 'LS51 actually runs in the Apple-1? In contrast to the 7451 or 74S51, the 74LS51 has three inputs on gate 1, but pin 11 and 12 of C8 are not connected.

- Pete

 

 

 

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Hi peo2000!

100%, I tried it too. So did 3x74LS00, 74LS02, 74LS04, 74LS08, 2x74LS10, 74LS32, 4x74161 (except D15 position).

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About the 7451 / 74LS51 difference ...

In post #4, peo2000 wrote:

 

"Uncle Bernie, are you sure that even the 'LS51 actually runs in the Apple-1? In contrast to the 7451 or 74S51, the 74LS51 has three inputs on gate 1, but pin 11 and 12 of C8 are not connected. "

 

Uncle Bernie answers:

 

Yes. I never write about IC (or other parts substitutions) which I haven't tried myself.

 

But you did answer your own question, didn't you ? See in your post#4:

 

"For whatever reason there is a 7451 at all, you can simply leave the two expansion inputs X and /X of a 7450 open."

 

... and exactly this is the dirty little secret why the 75LS51 works in the Apple-1. Open TTL inputs are "H", right ?

 

The technological reason (process technology, to be more exact) roots deeper. The standard TTLs use a multiemitter input stage while the LSTTL and STTL use a sort of DTL input stage made out of Schottky diodes - which cost very little chip area.

So they could use the expander inputs as additional AND gate inputs by just adding two Schottky diodes each - one of them being a clamp.

So unless the 74LS51 drops into a socket which actually uses the 7450 expander inputs, all is good !

(Of course, nit-pickers can now start complaining that open inputs in TTL should be avoided as it's not a reliable "H", but remember, the whole Apple-1 is riddled with floating TTL inputs, so it can't get worse).

 

- Uncle Bernie

 

 

 

 

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Uncle Bernie wrote:... Open

Uncle Bernie wrote:

... Open TTL inputs are "H", right ? ...

 

Thank you for the explanation, that makes it a lot clearer.

I can vaguely remember all these effects, but it was decades ago, and yes of course, we always tried to keep all input situations defined.

- Pete

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