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Date:      Sun, 8 Oct 1995 12:49:20 -0400
From:      dennis@etinc.com (dennis)
To:        Warner Losh <imp@village.org>
Cc:        hackers@freebsd.org
Subject:   Re: VLB Disk Controllers 
Message-ID:  <199510081649.MAA26550@etinc.com>

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>: I have had problems with vlb cards running above 33 MHZ.  The effect appears
>: like the loss of interrupts or extreme interrupt latency.  I have seen this
>: especially bad on amd 486dx2/80 (vlb clk 40mhz) and 486dx2/100 (vlb clk
50mhz).
>: I suspect the problem would exist on 486dx50 as well.
>
>The VLB spec doesn't go above 33MHz for two or more cards.  It goes,
>I'm told, to 50MHz for one card, but I've also heard that the 33MHz
>limit is hard, so I don't know what to believe.  Either way, it isn't
>surprizing that you've had problems with one i na 40MHz or 50MHz
>system, since you'll likely have a video card on that bus as well.
>
>I've had no problems with my VLB UltraStor 34F and either a no name
>ET4000 card, or an S3 #9 Lvl 11 at 33MHz.
>
It all depends on the MB, and even the tuning parameters are somewhat bogus
on some boards. Some MBs insert cycles so that any speed with work, but
they're not as fast. What you want is one where you can tune it precisely.
Most cards are designed to work at 40mhz at least, but thats based on
assumptions that may not hold true.

Dennis




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