Re: Z80 type differences?

["Alex Freed" <alexf@mirrow.com>]

"Knut Roll-Lund" <kr-lund@nogarbage.online.no> wrote in message
vfmdnedUGvx-G9LYRVnzvA@telenor.com">news:vfmdnedUGvx-G9LYRVnzvA@telenor.com...

> Z80A is 4MHz Z80B is 6MHZ so it depends on the clock, if there is a 2MHz
> or 4MHz clock Z80A should be fine.

This is correct.

> If the Z80 is using the Apple's ram I can't think
>   it could go faster than 2MHz (2MHz Z80 runs about the same speed as a
> 1MHz 6502 which uses both flanks.

z80 is running on "sincopated" clock. The effective clock is about 2 MHz,
but the bursts
are 3.5 MHz. The bottom line is the Z80A-CPU works perfectly.

Now the chances of the CPU beeing at fault here are relatively slim. If the
SoftCard "locks"
the computer even when no z80-specific software is loaded, it means that the
card incorrectly
holds the address, data or control (RDY and DMA) lines.

This is easy to test by just REMOVING the chips on the card. No harm will
come to anything
if these chips are missing:

1. U2 - 74LS05- controls the RDY and DMA. Pull it out, try the card. Still
locks?
2. U15 - 74LS373, U9 and U13 -74LS367 - data lines. Same thing.
3. U14, U16 and U17 -74LS367 - address lines. Same thing.

If Apple is not locked any more after removing (one by one!) the above chips
then we
know that the problem is either that (last pulled) chip or the one
controlling it.
Namely U3.

It is also possible that the chips are OK, but there is a short on the
board.


>
> -- 
> Knut
> (delete 'nogarbage.' for email)