non-CP/M Z80 board

[Holger Veit]

Roy J. Tellason schrieb:
> I'm not familiar with many 6800 designs,  but I was somewhat surprised to see 
> how limited some parts are,  like the 8085 in the "8085 Cookbook" where you 
> could really get away with very little.  OTOH,  the c64 doesn't use any 
> buffering _at all_ and yet the CPU in there seems to have little trouble 
> driving 3 ROM chips,  a set of 8 4164s,  plus all the peripherals.  I'm 
> guessing that the Z80 is probably somewhere in between,  and that the 
> datasheet probalby won't give me the whole story anyhow
The plain 6800 is not a good example to compare to the 8085; from its 
generation it rather relates to the 8080 three-chip system. Compare the 
8085 to the 6802, and you'll get a minimum 2-chip system 6802-6846 like 
the 8085-8355 pair. Using EPROMs rather than mask ROMs will expand the 
chipcount similarly in both ways.

The C64 does not use the 1st generation 6502, but rather a modified 8502 
CPU with different electrical specifications, so I guess it was 
explicitly designed to be able to drive its special C64 peripherals. It 
will, admittedly run into problems though with own extensions, e.g. 
replacements of the onboard ROMs and additions to the external expansion 
port.

The Z80 has also undergone several modifications throughout its first 
version, which may or may not resulted in higher fanout.

For all of them, one also has to consider that nowadays one won't use 
the circuitry that was used 20/30 years ago any longer, but use modern 
chips. This starts with low power HCT TTL, one will avoid DRAMs of that 
time like a plague now, using monolithic 512KB SRAMs instead, and maybe 
replace glue logic entirely with LP-CPLDs which can drive much more loads.

Regards

-- 
Holger