PET problem SOLVED!

From: Ethan Dicks <>
Date: Fri Aug 13 21:51:13 2004

On Fri, Aug 13, 2004 at 10:07:37PM -0400, Dave Dunfield wrote:
> Ethan, Dwight (and anyone else who helped),
> Thanks for all the help - I've solved it and it works!

> Turns out it was NOT the ROM's -- After my last post I got to thinking that the PET
> ROM's are probably fused type, and wouldn't go marginal like an EPROM...

They are mask-programmed, not fused like a bipolar PROM, but I agree...
when they fail, they tend to fail completely, not marginally (and I
_have_ had C= ROMs fail on me, but not as often as other problems,
like a C-64/SX-64 PLA, for example)

> so I focused on the RAM - sure enough, testing again, and I found that bit 1 of
> location $11 was stuck at zero

Wow... that's a really picky error, but given the code snippet, it
makes sense.

> Checking the Schematic, bit 1 of the low memory bank is U17 - pulled it, put in a
> socket (always!) and stuffed another 4116 - voila!!! it came right up!

> Also figured out the startup "noise" mystery (such unanswered questions "bug" me),
> turns out the PORTA bit 7, which is used for the DIAG input is also used to control
> the speaker (it was right on the schematic once I looked for it - duh) - pulling
> this line low disabled the speaker.

Ah... that's one of those things that doesn't appear in the older
motherboards, so I'm not surprised to find I had no idea that's
how it works.

Anyway... good job sleuthing it all out. Since you mentioned this
as a SuperPET, it reminds me that one of these days I'll have to
haul some of my NIB spares out of the attic and convert one of my
8032s into a SuperPET to see how they play. Fortunately there are
more resources out there for it now than there were ten years ago
when I got the stack of boards.


Ethan Dicks, A-130-S      Current South Pole Weather at 14-Aug-2004 02:40 Z
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Received on Fri Aug 13 2004 - 21:51:13 BST

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