I just got a ZX-81 kit (unassembled). I didn't
know this before, but the ZX
uses a custom 40-pin IC to perform certain I/O functions. Is this chip truly
"custom" or is it a relabeled Z80 support chip?
Yes, it is. Or better it is a semi cusom chip - like ASICs today.
The Basic design is a standard compilation of TTL functions, only
the last metal layer is custom. This was an early attempt by some
chip manufacturers to cut design costs - only this last layer had
to be designed for every customer. The technic was(is) called ULA
Unassigned(?) Logic Array. And thats also the name used by ZX-fans
for this Chip.
The ULA replaces something like 15 or 20 standard TTL bugs from the
ZX-80 design - the 81 is just a ZX-80 II :)
P.S.: Don't assemble it ... I assembled 5 kits several years ago -
and now I could cry for one :(
Ich denke, also bin ich, also gut