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Xerox 820 vs 820-II Disk Format

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leaknoil

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Joined
Aug 14, 2008
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425
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Central California
I have 820-I with a mess of disks. Some are originals for the 820-II. So far I haven't been able to read a single one that says it is specifically for the 820-II. Did they change disk formats ? I know they had a 8/16 upgrade for the -II that did MS-DOS. Some of these could be in that format but, it seems odd the things like Diagnostics and CP/M would be. It doesn't say CP/M-86 on it. Just CP/M 2.2 for the 820-II. Can a 820-II cp/m disk boot a 820-I ?

Even though I have about 15 original disks it wouldn't surprise me if they were just all bad by chance. Even out of all the ones for the 820-I I am getting a lot of bad disks.
 
I'd have to go digging out in the garage to be sure (and its too cold for that now), but I seem to remember that the 820-I used a single density WD 1771 disk controller with single-sided diskettes and the -II used a 1791 double-density controller and came with double sided diskettes. If that's true, than no, you won't be able to boot 820-II disks on a stock 820-I machine.
 
I was afraid of that. Oh well.

Since I have your attention I have another issue. It fails the memory diagnostics on the test disk. It still boots and runs but, if a program resides in that memory it tends to crash obviously. Using the monitor commands to play with individual locations it looks like bit 2 of address 8000-BFFF is stuck on. ie.. putting a 01 into any location in the range reads back as 03. All other bits seem fine.

Anyone want to take a look at http://www.bitsavers.org/pdf/xerox/820/Xerox_820_Schematics.pdf and make a guess which 4116 it might be ? There isn't enough room in the case to get at anything with the system assembled.

Oh and is there a way to interrupt an operation on a bad disk after you get the DISK ERROR message ? Seems like pressing any key just tries the operation again. I just want to give up and go back to the system disk without having to reboot.
 
Last edited:
leaknoil said:
I was afraid of that. Oh well.

Since I have your attention I have another issue. It fails the memory diagnostics on the test disk. It still boots and runs but, if a program resides in that memory it tends to crash obviously. Using the monitor commands to play with individual locations it looks like bit 2 of address 8000-BFFF is stuck on. ie.. putting a 01 into any location in the range reads back as 03. All other bits seem fine.
Click to expand...

That would be bit 1--bits are numbered from 0. Given that, it looks as if U23 might be the one.
 
Terry Yager said:
Oh ghawd! Why'd ya hafta go and remind me?!?!? Now I'm gonna have nightmares tonight too! :roll:

--T
Click to expand...

It is kind of amazing these didn't just cook themselves. No fans or cooling of any kind other than the small vents at top. All packed into a very small thick case with a CRT. Everything inside is sort of covered in sooty grime but, other than the ram seems to work fine.
 
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