Tarbell is making me insane

Grant Stockly grant at stockly.com
Sun Nov 11 16:21:32 CST 2007

Tarbell Update

> >> > Also sounds like oneshot problems.  Check cpu timing.  Even small timing
> >> > errors tend to magnify bus noise issues and incompability problems.
> >>(Snip)
> >> > Also HEAT.  That thing despite a very heafty noisy fan and cover mods
> >> > didn't like heat.  FYI: the timing of the oneshots drifts with heating!

I can't speak for the exact combination of the Tarbell, but the 
system has been running all day long with:
-16k static card
-8k static card
-4k static card
-1k ROM
-8k Byte saver
-Tarbell FD

It has been running solid a 4K ROM monitor from Dave 
Dunfield.  Floppy interface still doesn't work on either card, and 
these cards were tested by someone else on an IMSAI with an Altair CPU card.

>Some Z80 cards complicate the issue.  They have diffent timing and at 4mhz
>some ram MAY NOT be fast enough.  Either slow the z80 to 2mhz or more wait
>states from ram.

The card I have has a switch for 2MHz.  The tarbell with the reduced 
clock speed will not work at anything higher like you suggested.  I 
have confirmed (from the person who shipped me the tarbell cards) 
that the tarbell card with a reduced clock speed works normally with 
a Z80 at 2MHz.

I attached a logic analyzer to the drive and watched all of the 
signals.  Every thing looks normal to me.  I hooked it to a 5.25" 
drive and then to a 3.5" drive.  The drives are sending everything 
that they should.

>Shows how rough the bus timing and noise can be.  Is that a one piece
>backplane of the two sided variety or the one of the earlier Altair
>4 slot chains or single sided backplane?  The reason is the earlier

2x4 slot motherboards, but all of the wires are the same length 
(there was a tip on that to reduce noise)

I'm going to let the thing heat soak and then check the bus with an 
oscilloscope and the CPU clock.

You know, I'm worried that the reliability issues aren't related to 
the Altair, but to a heavily modified tarbell card trying to write a 
Tandon SSSD 5.25" onto a 3.5" disk???

Would any of you like to see the data coming out of the disk 
drive?  I can upload the logic analyzer data to my web site...

I will never be happy until I find the problem.  I'd like to think 
that with all of the information we have that I can make the Altair 
as reliable as any other S-100 system.  I have collected a lot of 
errata and have applied it all to my system...  That's why I won't 
give up with this setup.  : )


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