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Ohio Scientific 400 "Superboard" with Motorola 6800 and MOS 6501

glitch

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Feb 1, 2010
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Central VA
Well, we finally built up the original OSI 400 "Superboard" from which the reproductions were made. It was built up to support the Motorola 6800 instead of the 6502:



Why would anyone do that? Because we had a couple of MOS 6501s to test!







It does work:



The OSI 400 is one of the only things that explicitly supports the MOS 6501 -- it is discussed in the manual as being an option. There were a few goofs, trying to follow the manual on setting up for the 6800/6501. Planning on doing a build writeup detailing the issues we ran into.
 
I cleaned up the 400 board some:



Built the RS-232 level shifters from the manual, adjusted the values for the 555 bitrate oscillator to 153.6 kHz (9600 bps). Tacked down all the blue wires with epoxy.

Backside current state:



Level shifters/bitrate clock detail:



I used some old TO-106 package transistors for the level shifters, the one on the left is NPN, one on the right is PNP. To get 9600 bps, use a 4.7K as the manual says, then a 47K right above the capacitor. Use a 82pF capacitor -- the ceramic on there is unacceptable as it has a horrible tempco, just putting your finger on it will swing the frequency around 4 kHz in around 15 seconds! I've got some polystyrene caps on order for that, but it does work.

I got onboard RAM working, turned out to be a cracked pin on one of the 74LS03s. I am going to port Martin Eberhard's Altair 680 RAM tester to the 400 board to fully check it out.
 
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