23 Jun
1997
23 Jun
'97
3:13 p.m.
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
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!
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
possible.
Tim.
--
-tony
ard12(a)eng.cam.ac.uk
The gates in my computer are AND,OR and NOT, not Bill