PDA

View Full Version : Xerox 820 vs 820-II Disk Format



leaknoil
February 12th, 2012, 01:07 PM
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.

jharre
February 13th, 2012, 08:06 PM
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.

Chuck(G)
February 13th, 2012, 08:07 PM
That's exactly correct. The 820-II can read 820-I floppies but not vice-versa.

leaknoil
February 23rd, 2012, 01:37 PM
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.

Chuck(G)
February 23rd, 2012, 02:27 PM
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.

That would be bit 1--bits are numbered from 0. Given that, it looks as if U23 might be the one.

leaknoil
February 23rd, 2012, 03:27 PM
That would be bit 1--bits are numbered from 0. Given that, it looks as if U23 might be the one.

Sorry. I knew that. I'll give it a try. Thanks again. That also helped me understand the schematic better. Now I see how it works. Sort of obvious now. Doh.

Terry Yager
February 23rd, 2012, 04:35 PM
There isn't enough room in the case to get at anything with the system assembled.

Oh ghawd! Why'd ya hafta go and remind me?!?!? Now I'm gonna have nightmares tonight too! :roll:

--T

leaknoil
February 23rd, 2012, 07:22 PM
Oh ghawd! Why'd ya hafta go and remind me?!?!? Now I'm gonna have nightmares tonight too! :roll:

--T

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.

leaknoil
February 25th, 2012, 09:02 AM
That would be bit 1--bits are numbered from 0. Given that, it looks as if U23 might be the one.

That was it. Passes all the memory tests now. Thanks again.