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800XL with Mystery (to me) Daughterboard

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I was lucky enough to win a small haul of Atari 8 Bit stuff on Ebay a few weeks ago (I've been using 8 bits since the 80's), which included a Atari 65XE, 800XL and some cassette drives.

My first observation of the 800XL was that it had a knob on the back....strange, never seen that before.


So I plug it in and all I got was a blank screen.  I tried the knob in both of the available positions and it didn't make any difference.

So opened it up and saw a bunch of yellow wires going to the knob as the back from under the RF shielding.

Took the RF shield off, and there was a daughterboard with 3 chips on it.

I quickly established that one was the OS chip, but no idea what the other 2 are.  


It looks like one says "Ultra Speed" on it.


There was also a piece of paper in there that had a small wiring diagram that looks like it was of the SIO port.


But apart from the yellow wires, there are 2 blue and white wires that aren't connected to anything, and I believe the wiring diagram is related to them.

Anyone know what this is?...it's a very elaborate bit of soldering on the board.

I have removed it from the computer now, and put the OS chip into the socket on the mainboard, and now the 800XL boots up just fine.

Any info would be great....and I guess someone here may even recognise it















Edited by ironeagleuk

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looks like a home made multi OS selector.  the rotary switch probably had 4 or more positions.  3 positions should have booted 3 different OS chips.  One (modified) for Ultra Speed SIO routines I'm guessing.  Unsure about the other.... maybe OSB for quick compatibility with non XL compliant software ?


You should be able to swap the OS chip out for the others, and maybe you can determine something more about them that way.




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Very homebrew looking indeed but high speed SIO is a great mod!

Is the 800XL an 800XL'F' model? i.e. it uses the Freddie chip for memory management? I only ask because i spotted the mysterious plastic blanking plate at the back of the case that the F models seem to have.

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