Tarbell is making me insane

Allison ajp166 at bellatlantic.net
Wed Nov 7 09:53:34 CST 2007

>Subject: Tarbell is making me insane
>   From: Grant Stockly <grant at stockly.com>
>   Date: Tue, 06 Nov 2007 23:07:13 -0900
>     To: cctalk at classiccmp.org
>I'm about to go insane.
>Here is what I have:
>-Fake Altair (rock solid in everything but tarbell)
>-Tarbel 1011D
>-Tarbell 1011A, which SEEMS to be better than the 1011D.
>-4xMITS 16MCS 16k memory cards
>Anyway...it doesn't work at all if I have a 16k memory card and an 8k 
>memory card.  The 1011A works best (gets the furtherest in all 
>tests).  If I'm lucky it asks me how many disks I have!
>It also doesn't work with 3 seals 8k cards.
>When I put in a CompuPro active terminator on the end of the bus, the 
>computer goes crazy and doesn't do anything.  Installing an 
>"Industrial Micro Systems" terminator from 1980 it changes 
>operational characteristics, but still doesn't quite do it.
>I had been trying to make the stupid thing work for months, on and off...
>Well, I decided to stop trying to limit the number of cards on the 
>bus.  I put in 5 8k seals cards and the stupid thing boots and runs 
>commands.  Before while loading BDOS it would most of the time report 
>corrupted or missing sectors.  Or just print trash.  Now I can run 
>DISKDMP and programs like that.
>After working flawlessly for a few minutes its back to crazy.  I can 
>single step through the tarbel boot program and see it going crazy...  : (
>I'd like the thing to be rock solid with minimal memory cards.
>Can anyone think of WHY the tarbel card is so much trouble?  I burn 
>EPROMs all day long with this Altair.  It takes 3 minutes to burn a 
>1702 and I've never had one fail!  I can also leave it playing music 
>for all night long with no problems!!!

Welcome to S100.

First ANY CARD that uses wait (many disk controllers stall the cpu
waiting for data) will tend to corrupt Dram depening on the DRAM
card and timing.  I found 884MCDs problematic until I converted them 
to a pin compatable static rams.  The S4Ks sorta worked but not with DMA.

I don't have info on tarbel 1011 seris FDC. The schematic on hand I 
could comment more.  However 8" DD with 2mhz 8080 is very difficult 
to do as single density is already pushing the 8080 through some very 
tight software loops.

Of all the cards the SEALs 8k and the PT 8Ks were about the best.

Also sounds like oneshot problems.  Check cpu timing.  Even small timing
errors tend to magnify bus noise issues and incompability problems.

Some cards were timing incompatable due to how they decoded status 
signals and tended to be either sensitive to bus noise or cause
bus noise.

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!

In general It sounds like the usual problems I had with the Altair
prior to retirement.  It was always conditionally stable, make a change
start over again.   It Improved when I replace the CPU one shot clock
with an 8224, better still with a WAMCO "quiet mother" backplane and 
improved more when I went to a Z80 card (NS* ZPB).  near the end the only
thing stock was the front pannel (even the connecting leads were 
shortend and extra grounds added).

The 8800BT has none of these problems, It has both tarbell and MITS floppy


>I need help!!!  : (

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