Tarbell is making me insane - test-drb@ccmp.vtda.org

8 Nov 2007

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Subject: Re: Tarbell is making me insane
   From: Grant Stockly <grant at stockly.com>
   Date: Wed, 07 Nov 2007 23:35:01 -0900
     To: "General Discussion: On-Topic Posts Only" <cctech at
classiccmp.org>

 Changing the crystal would half the data rates
making it right for
5.25" floppy.

One of the modes all the contrllers that use hang mode (wait), is
sector never found. Wrong clock rate will insure that. 
I was also considering that at 2MHz instead of 4 the FDC controller 
will be operating close to its limits and may need the wait 
state.  What was happening was probably a missed wait state. 
More  like the data rate for the board was halved 250kbits/sec
and the data was never being read.  That will hang the system as 
the FDC is waiting for the data and at the wrong rate it will 
never arrive.  The Clock crystal is used to derive the data rate 
for reading and writing.

...
  Both work.  The
logic however uses the 8T97 and tristates it when
not needed (board not addressed) which is a "legal" way to do it
though it seems like more effort.

Of course using high is active and mixing than with low is active was
a dumb thing too.  Even Tristate asserts a better low than a high. 
The front panel buffer is hard wired on, so the Tarbell was shorting 
out the panels XRDY. 
That too.  On front pannelless systems thats not an issue.

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I reviewed the 1k memory card and it does tristate it like you 
said.  My problem with the 1k card way back then was that the MITS 
documentation puts both on the PRDY line.  In my two systems this was 
no good.  The transients would corrupt the bus cycle.  This was found 
when working with an altair laying on a wooden table.  It wasn't in a 
metal case.  I never tried putting both on the PRDY line when it was 
in an enclosure.  I imagine it would have worked better in the case 
since I've never read any errata on it, but I wanted to play it safe. 
It would ahve worked btter is the rams used wer 600ns rather than 1uS
and the wate states never needed.

...
 If you want to talk about media abuse...1.4MB disk with
the HD hole 
covered, formatted to 71k  : ) 
That works.

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One of my problems MIGHT have to do with the 16v and 8v supplies not 
turning on at the same time.  I will have to investigate that more... 
Why do they not turn on at the same time?  All the altairs I know of 
the switch was on the AC side and it all came up soon as AC was applied.

Allison