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Hi Phil, You are correct, only one /CS should be active(low) for any given access. CS0 should be high for anything above $03FF. That suggests a faulty 74lS138 at U26. I'm not sure what to make of the write issue. It almost seems as if the clock bit for a previously written track (at a very slightly ...

Jeff, that's great! The nice thing about these machines is how straightforward they are to work on.

Dave

Yes, the C3 series had an option for either a 29MB or a 74 MB hard disk. Those were huge at the time. The "H" option used different boot code from the "D" option, since the hard drives were controlled by a different hardware subsystem from the floppy controller. Is there any information available o...

Yes, the C3 series had an option for either a 29MB or a 74 MB hard disk. Those were huge at the time. The "H" option used different boot code from the "D" option, since the hard drives were controlled by a different hardware subsystem from the floppy controller.

Jeff wrote:Does anyone know what the 'H' means in the H/D/M? prompt?
/Jeff

It stands for Hard disk.

Great!

I found this reference to the OSI 541 board. This confirms it's a high-res board, providing 256x512 graphics, overlaid on the character graphics. Apparently there was a similar board for the C1P providing 256x256 bit-addressable graphics. Jeff, I think your system was either upgraded with a 541 boar...

Hi Jeff, For some reason, I originally thought those were 2102 chips for a total of 6K, but as you point out, they are 2114s for a total of 24K. That puts a high-res option back on the table. The question is, do the address and data lines go anywhere else besides the system bus? I believe there is a...

Jeff, that's great news--well done.

Dave

I understand the OSI 542 keyboard and the 600 have the same layout but they are actually scanned differently (ie: a key down is "1" on 542 but "0" on the superboard... or perhaps the other way around). Steve Yes, the C2/4/8 machines uses inverting buffers, and the 600 uses non-inverting buffers, so...