[vcf-midatlantic] Tarbell DD controller troubleshooting

Herb Johnson hjohnson at retrotechnology.info
Fri Aug 11 13:16:16 EDT 2017

Rich Cini said "not letting it reset" means the S-100 system works 
without the board, but doesn't work with the non-working board inserted. 
Rich, I think that means the board crashes the bus, not necessarily that 
it screws up reset.

On the "no data on select" board; I think you are telling me, you 
confirmed chip-select on say the FDC chip is properly enabled. But you 
say changing bus buffer chips and the FDC chip doesn't change your 
results. It may be inconsistent bus timing.

S-100 bus timing depends on the specific CPU board (and front panel) 
versus the board under test. If you've never used a Tarbell DD board in 
that S-100 box, and two don't work, and the data you read is a 
persistent incorrect value; mismatched timing is a possibility. S-100 
control/status signals also depend on chip delays and even R/C circuits 
which age poorly.

  I see your point in using a bus analyzer, to look at all relevant 
signals. I look at signals with a 2-channel oscope, one channel often on 
the bus clock or other timing source.

For home-gamers reading this: note that I'm saying that S-100 is not a 
single, consistent "standard". S-100 cards don't all play well with each 
other. I claim three flavors of "S-100" bus, on my Web site; early 
Altair, S-100 MITS/Altair/others, and IEEE-696.

"It worked previously" points to corrosion problems too. Look for TI 
brand chips which have blackened pins; they seem to fail after a period 
of recent use. I chased this down with my Ithaca Intersystems repair. 
That happens across S-100 cards with socketed chips.


Rich, contact me privately for further discussion. Chasing chips is 
boring for many people; they can read your results on your Web pages.


Herbert R. Johnson, New Jersey in the USA
http://www.retrotechnology.com OR .net
preserve, recover, restore 1970's computing
email: hjohnson AT retrotechnology DOT com
or try later herbjohnson AT retrotechnology DOT info

More information about the vcf-midatlantic mailing list