Apple II hardware design

From: A.R. Duell <>
Date: Mon Jun 23 22:13:41 1997

> > Well, lets see... Does anyone know when the 74LS68x series came out? Were
> > they around when the Apple ][ was current. PALs were sort-of current, and
> > I guess a 14L4 would be an ideal address decoder.
> Ah - if you think of 74LS68x and 14L4's as "current" with the introduction

Well, I knew the PALs were a little later (1980?) - the first PERQ had no
PALs at all - only PROMs, TTL, ECL, RAMs and Z80 stuff. Later ones were
stuffed with PALs.

When did the 74LS68x come out. I have no idea.

> > If you insist I stick to 'classic' TTL then I'd want a 74LS133 13 input
> > NAND gate, and a couple of 74LS04 inverters. That would decode just about
> > any combination of 13 address lines. If you want all 16, then add a
> > 74LS138 for a total of 4 chip max (and you might get away with only one
> > '04 if you're lucky.
> Remember, 4 more chips increases the chip count on a Disk ][ by 50
> percent!

So? I think of it as 4 more cheap chips, not a 50% increase in hardware.
And yes I do realise that the cost of chips is hardly the end of the story
- board area and layout costs a lot more than TTL.

> If you don't like having the traditional slot addresses predecoded,
> you've always been able to do your own decoding. The resulting
> cards won't be compatible with the existing scheme of slot
> addressing, so you better write your own operating systems and
> languages so that the users can use your new devices :-)

I dislike the idea of geographical addressing which limits the number of
slots you can have, and means the user has to know which slot things are
in. I didn't like the bank-switching scheme for the ROMs on the I/O cards,
but as address space was tight (although a lot was wasted by those
addressable latches used as 'soft switches), I guess nothing else was

> Tim.

The gates in my computer are AND,OR and NOT, not Bill
Received on Mon Jun 23 1997 - 22:13:41 BST

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