Reading Atari ROMs

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adrm
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Reading Atari ROMs

Post by adrm » Tue Oct 16, 2018 11:37 am

I posted this on the atari-forum.com site, but their 8-bit section is hidden at the bottom of the page, and I don't expect a reply, so I'll try here as well:

I have a faulty Atari XEGS that I'm trying to fix. I have already replaced two RAM ICs, as they were clearly faulty.
But the machine will still not show anything but a black screen, and no sound appears if I try to run the built in Sound test.

No chips are hot now that the RAM has been replaced, and the scope doesn't show me any obvious culprits ... yet.

I have un-soldered the ROM, and I'm attempting to read it using a TL866-II Plus EPROM reader/writer.

Problems:
1)
I always get a "pin detection type error" pointing to pin 1.
This might not necessarily point to a problem in itself and disabling "pin detection" in the software bypasses this.
2)
Whatever 27C256 type EPROM device (which is interchangeable with the XEGS ROM) I have selected in the reader software, it will only read 100% "00 bytes" or only "08 bytes".
I.e. either the ROM is faulty or the TL866 is not reading it correctly.

Question:
Has anyone managed to read these ROMs using the TL866-II ?
(I can read other devices on the TL866, e.g. 27128 EPROMS)
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tricky
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Re: Reading Atari ROMs

Post by tricky » Tue Oct 16, 2018 12:40 pm

I just had a quick look on Atari age at this thread.
Do you know if the MMU GAL is OK?
I also don't see how you can put four copies of an 8K OS in an 8K EPROM, but they say it should give the self test!
Sorry if I am referring to something that you have already investigated.

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adrm
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Re: Reading Atari ROMs

Post by adrm » Tue Oct 16, 2018 2:14 pm

tricky wrote:
Tue Oct 16, 2018 12:40 pm
I just had a quick look on Atari age at this thread.
Do you know if the MMU GAL is OK?
I also don't see how you can put four copies of an 8K OS in an 8K EPROM, but they say it should give the self test!
Sorry if I am referring to something that you have already investigated.
Not sure if the MMU is ok yet. I had a spare 6502C and swapped that in, but no extras of the other major Atari ICs.
At this moment I'm trying to look at the simpler components to determine if there is an obvious problem.

I've read that the ROM chip is notorious for just suddenly dying, so I felt it was reasonable to try to read it. Problem is: I'm not sure if I can't do that because the IC is busted, or because the TL866 is not working well with this chip.

The XEGS ROM contains 3 parts: Missile Command game; BASIC; OS - 8KB; 8KB; 16KB respectively (as far as I know)
So putting the OS part into a 27128 should be possible, as long as you trick the Atari into seeing it in the correct address into the correct memory space.

I'm wondering about trying to examine the ROM's output using at Logic Analyzer at boot up, but that might not give a meaningful result as the problem could be elsewhere and the ROM might not be accessed at all?
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Tore

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adrm
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Re: Reading Atari ROMs

Post by adrm » Tue Oct 16, 2018 7:54 pm

Quick update.

I only had 16K EPROMS so I flashed one containing only the OS part from the XEGS' original ROM, i.e. without BASIC and Missile Command.
Seems to work fine now.

This unit seemed to suffer from the 2 most common problems for this machine:
- Faulty RAM ICs
- Faulty ROM chip
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Prime
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Re: Reading Atari ROMs

Post by Prime » Wed Oct 17, 2018 12:08 am

I think faulty RAM ICs is one of the most common faults on any machine of this era :) :)

IIRC the XEGS I repaired for Arcadian had another fault I think that was the one where the 6502C has also failed. so made up a board to convert a standard 6502....basically a copy of the circuit from the original 400 and 800 machines.

Glad you got yours fixed tho.

Cheers.

Phill.

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