[kragen]

Always exciting to see a new Toledo post, but this one is especially inspiring, because it talks about the author's errors that you will probably make too if you don't read the post, and tells how to overcome them. And the activity described is the highly practical activity of designing a PCB this year with the best current free software and getting it built and debugged; the fact that it's an ISA board for a Transputer is fun but not central to the problem-solving process.

I wonder why he used the 74LS00 family instead of 74HCT00, even if he really needed the TTL thresholds? I forget if ISA even requires TTL levels. Is that a question of nostalgia, or is there a practical advantage of TTL over TTL-compatible CMOS in this context that I'm unaware of?

[nanochess]

The practical advantage is that the 74LS chips are available in several corner stores at 25 or 35 cents each one, and I preferred the logic to be the same family.

You cannot choose the source, these come mixed from ST, TI, and other manufacturers. I preferred the laser-engraved ones instead of the white ink ones just to have an uniform look.

The crystal oscillator needs something faster so it requires 74F04, and the link communication buffer requires 74F244 or 74AS244. These are more expensive, the 74F are 2 dollars each chip, and the 74AS are 4 dollars each chip.

[kragen]

Oh! I usually think of availability as an advantage for 74HC (maybe not 74HCT) over 74LS, but maybe that's not the case where you are. Local availability is a real advantage.

The electronics parts stores in my town (Morón), which are several blocks from my house, have a fairly limited part selection, mostly for repair purposes. So part availability is a very significant concern. I was shocked to find last month that one of them didn't even have a TL431! But another one on the same block did.

I thought I'd check Digi-Key, but it seems like 74AS244 is even more than US$4 there in onesies: https://www.digikey.com/en/products/detail/texas-instruments.... However, a CMOS 74AHC244 (nominally something like 5.5ns to the AS244's 6.2ns) is only 27¢: https://www.digikey.com/en/products/detail/texas-instruments...

At Digi-Key, the CMOS parts have better availability in this case, but that says nothing about the availability at the corner store.

Thank you for explaining!