Tarbell is making me insane
ajp166 at bellatlantic.net
Thu Nov 8 17:01:48 CST 2007
>Subject: Re: Tarbell is making me insane
> From: Grant Stockly <grant at stockly.com>
> Date: Thu, 08 Nov 2007 11:02:00 -0900
> To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at classiccmp.org>
>At 10:11 AM 11/8/2007, you wrote:
>>On Wednesday 07 November 2007 10:53, Allison wrote:
>> > Also sounds like oneshot problems. Check cpu timing. Even small timing
>> > errors tend to magnify bus noise issues and incompability problems.
>> > 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!
>>This probably explains a lot why I encounter 9602 (and the LS variant) fairly
>>often in stuff that's otherwise full of the standard 74xx (or LS) parts.
>>I remember back in 1978 trying (unsuccessfully) to make an interface
>>a dual cassette deck that we had on trial to work with the H11 system I'd
>>built for this company. The fella on the phone warned me to stay away from
>>the '123 parts for just this sort of reason...
>Its not the one shots... I installed a Cromenco ZPU card and it also crashes.
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.
FYI: the Seals 8k [most 2102 basd cards] cards are marginal for Z80
above about 2.5mhz.
>I really wanted to get this setup working. In my experience, the
>Altair disk subsystem with a half dozen one shots is more reliable
>and predictable to the Tarbell (hehehe). And taht experience was
>with a CPU card that had one shots so out of spec according to the
>intel data sheets the 8080 should not run.
>Now I have a CPU card that has been tweaked to have perfect CPU clock
>timing. Nothing. : (
Also make sure the crystal osc is really running at 2mhz. Mine would
sometimes take off at 4 or 6mhz until I dumped it for 8224.
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
two styles were noisy at best and terminators didn't work well on them.
Replaced the backplne with WAMCO unit and the problem improved and
terminations worked. The front pannel still induced a lot of ringing
on the bus.
>My next guess is going to be switching the disk drive to run off of
>power from the Altair. The disk drive is getting about 5.25v from
>the AT computer power supply and the S-100 boards are regulating
>their 5v to 4.95v. I really don't think that could be a problem, but
Thats not it. Do make sure the drive and it's DC have a good common
>I'm willing to investigate anything right now. I am going to give
>the manual a close look and make sure there aren't Altair specific
>wiring instructions that were missed. I just can't figure out why it
>is going crazy! I might have to get out the oscilloscope.
You might. One thing to check. Is the raw BUS voltages at least 8V
and 16V?? I when through two sets of PS mods to get enough power
for a 48K of 8K 2102 rams, a VDM1, Altair PIO, Altair SIO plus
eventually a NS* MDS.
>I installed all of the boards in a "vintage" Altair with less luck.
Thats why mine is retired. I'd feel different if it were an 8800B.
>Grrr... This is the 3rd Tarbell card I've used trying to get
>something good. The cards are modified by someone, work PERFECT in
>his IMSAI with one of my reproduction 8080 CPU cards, and mailed to
>me. He found that he needed a terminator to make it work, so I
>installed a terminator that he sent me. The terminator only made
It could be the front pannel doing something to you.
Now your getting a feel for what life was like for an Altair owning
early adoptor. I still really feel that the hardware issues I fought
with delayed my developing software efforts. It was when I went to the
NS* and Netronics Explorer 8085 that I stopped chasing hardware and
started working with higher level languages and system performance.
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