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 XT 68k 
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Joined: Sat Feb 02, 2013 9:40 am
Posts: 1699
Location: Canada
A photo of a 68000 on a board used in an XT computer. The 68000 was run at 7.16 MHz while the XT only ran at 4.77MHz. The interface was through a 1kB dual ported RAM. The 68k had 512k DRAM and 64kB EPROM space.
Sorry about the poor picture quality.

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xt68k.jpg


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Robert Finch http://www.finitron.ca


Sat Jun 14, 2014 8:24 am WWW

Joined: Sat Feb 02, 2013 9:40 am
Posts: 1699
Location: Canada
Here is a rough schematic for the project.
Attachment:
PC68001.gif


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Robert Finch http://www.finitron.ca


Sat Jun 14, 2014 9:03 am WWW

Joined: Wed Apr 24, 2013 9:40 pm
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Location: Huntsville, AL
Rob:

Thanks for posting the project.

Perhaps I'm missing something, but I can't make out where your connection/driver for the nG pin of the DRAMs comes from. Is there a missing gate?

Michael A.

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Michael A.


Sat Jun 14, 2014 2:46 pm

Joined: Sat Feb 02, 2013 9:40 am
Posts: 1699
Location: Canada
Quote:
Perhaps I'm missing something, but I can't make out where your connection/driver for the nG pin of the DRAMs comes from. Is there a missing gate?


Yes, there's likely a gate (or two) missing off the schematic (well I said rough schematic). I may have gated R/W with CS from the 138. The DRAM doesn't output until it see RAS/CAS, and the write signal is already determined (no bus contention), so the pin was maybe grounded. I dismantled the board a few years ago so I can't check.

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Robert Finch http://www.finitron.ca


Sat Jun 14, 2014 9:00 pm WWW
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Joined: Tue Jan 15, 2013 5:43 am
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Cool!

What's the chip that does the DRAM multiplexing? (The DRAM interface topic came up recently as part of mercury0x000d's topic, here.)

-- Jeff

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http://LaughtonElectronics.com


Sun Jun 15, 2014 6:53 pm WWW

Joined: Sat Feb 02, 2013 9:40 am
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Location: Canada
Quote:
What's the chip that does the DRAM multiplexing? (The DRAM interface topic came up recently as part of mercury0x000d's topic, here.)


The chip was a DP8409 by INS I think. I picked it up at the surplus store. I used to have photocopied specs for it around somewhere. That was before the time of the internet. It has several modes of operation and I think the capacity to do hidden refresh. It's an older chip, I doubt it would support the new-fangled DRAMS. But for a retro project it's maybe better than LSTTL muxes. Nostalgic moment here.

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Robert Finch http://www.finitron.ca


Mon Jun 16, 2014 7:29 am WWW
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