Re: Mousecard HW question...

[mjmahon@aol.com (Michael J. Mahon)]

Roger Johnstone replied:

>In article <3D08055C.4A2BD15@computer.org>, Randall Frank 
><randall-frank@computer.org> wrote:
>
<snip>
>> Yes, I reseated the socketed chips and tried other slots
>> (including one I know works) with the same results.  Looks
>> like I will need to work this one backward from first
>> principles...
>>
>> Is there any other component on this board that I should
>> suspect over others?
>>
>>
>> BTW: I really appreciate the input, even if I never get
>> this card up and running, thanks!
>
>Something simple to try would be to swap the EPROM with a known good one of
>the same type from another card. It won't matter what's in the EPROM as long
>as you don't activate the card, but you should be able to read the data
>stored in it if the rest of the card is OK.

Good idea, Roger.  That will eliminate the EPROM itself as the
problem.

If you still read unrepeatable data from another EPROM, then
there are really only two other possibilities:  1) the one or two
gates involved in getting -IOSELECT and R/-W to the EPROM
chip select/enable inputs, and 2) a possible octal bus driver,
for example a 74LS245 (and its enables--but they are essentially
the same as the EPROM enables).

These ICs rarely fail, but they are connected to the edge
connector, so anything from ESD to powered insertion is
a possibility.

-michael

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