Re: [N8VEM-S100:3402] Re: Help searching a component (AM8224-4DC)

[Fabio Battaglia <hkzl...@gmail.com>]

On 30/04/14 20:17, monahanz wrote:
> Fabio have you tried 36MHz, sometimes you can get away with higher
> clock speeds -- particulary later edition chips.
> John
>
Actually, I did, with an 74LS424. I get a clock output of about ~3.6Mhz
and the circuitry on the board _seems_ to operate, but I fear there
might some out-of-synch signals, because no matter what I try, I cannot
get the EPROM enable circuit to work. I'm waiting for some 8224 from
USA, I'll check those too.
On the rev.2 the ROM enable circuit is very simple, attaching the
schematics (I take NO credits for this, John King from Soundfarm sent
this scan to me).

As far as I understand, the EPROM /CE is enabled by IC29, a 74LS138,
which is used to select the address at which the data appears.
The problem is that the signals at pin 4, 5 and 6 of IC29 never match
the correct combination to enable the outputs (none of the outputs ever
gets to low!). I sniffed the signal lines and this proved to be true,
I'm attaching a screenshot (disregard a signal being called A11, it's
actually the input on pin 5).

I already tried replacing the Z80 and all the ICs which are part of the
/MEMR circuit, no results.
Any idea beside some clock problems?

Thanks,
Fabio

Attachment: Z80 Rev2 Schematic.jpg
Description: JPEG image

Attachment: micro.png
Description: PNG image