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To: dkelly@hiwaay.net
cc: freebsd-hackers@freebsd.org
Subject: Re: Reliable probing techiniques for isa bus? 
In-reply-to: Your message of "Thu, 09 Oct 1997 22:05:13 EST."
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Date: Fri, 10 Oct 1997 13:31:59 +0930
From: Mike Smith <mike@smith.net.au>
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> > there is no answer..
> > yes the ports will read ff
> > they will also read ff for any other device that does not use them :)
> 
> I don't think you can count on bus float to read as 0xff. Its a tri-state
> bus, not an open collector bus. If nothing is driving it then anything
> is possible.
> 
> I don't have an ISA book handy, but I have seen pullup resistors in places
> that suggest bus termination. But if they are terminators they are just
> as likely to be both pull-up and pull-down at the same time.

Please, not this argument again.  If you are going to post on hardware 
topics, please check your references first.  For ISA, I recommend 
Solari's "AT Bus Design".

The short answer is that there *is* a standard for ISA bus termination, 
and it involves fairly small pullup resistors.  Providing you're not 
going back-to-back with another ISA cycle that won't have read all 
high, you can be prattymuch assured of reading all ones.

mike