[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: Apple floppy adaptor. A little update
Michael J. Mahon wrote:
> Tristan Mumford wrote:
>> G'day!
>>
>> Today I had a feeling, which I acted upon. I wondered if the little short
>> the adaptor board had didn't cause more damage than I thought. It seems
>> it may well have.
>>
>> Just to find out, the Atlas 8 was plugged in to do a few tests. It can
>> read perfectly, as expected. However trying to format a disk results in
>> the same failure to write I have been seeing.
>>
>> Based on the assumption* that the short was from -12v to WREQ I traced
>> the lines to what seems to be an open-collector 74LS06 (M53206P) on the
>> controller.
>
> Usually, it's an 'LS05--but still an OC inverter.
So, should I replace it with an '06 or an '05?
>
>> Holding my amazing LED and resistor logic probe on the WREQ line and
>> attempting to perform a write (ProDOS format) results in no difference in
>> the logic high state.
-snip-
>> One odd thing to note is the IC in question doesn't seem to have any
>> continuity between +5 and the signal line (when not powered) as I would
>> have thought for this problem.
>> * Visually it looked like a solderbridge was between WREQ and -12v. Also
>> that was where I had just done an alteration to add a line to monitor
>> WREQ to know when to use the disk writing code in the uC. So it seems
>> logical that this was the issue.
>
> If it's an open collector output, then I wouldn't expect any
> connection with +5. The only +5 it sees is from an external
> pullup.
>
> More likely, it's lost its ability to pull the line *down*, because
> its output transistor/metal is blown.
I see. Oh well hopefully the presumed cause is correct and it's only one
commodity IC to replace.
Incidentally when I built the hideous auntieboard ( well it's bigger and has
more components than the mainboard ) I made sure to keep the pin
assignments the same. So only the fuses and delay in my source need to be
changed to let it recompile on the intended target uC. So hopefully the
project will still be cheap enough for everyone :)
Tristan.
--
-----> http://members.dodo.com.au/~izabellion1/tristan/index.html <-----
===== It's not pretty, it's not great, but it is mine. =====