Date: Fri, 10 Oct 1997 13:31:59 +0930 From: Mike Smith <mike@smith.net.au> To: dkelly@hiwaay.net Cc: freebsd-hackers@freebsd.org Subject: Re: Reliable probing techiniques for isa bus? Message-ID: <199710100402.NAA00796@word.smith.net.au> In-Reply-To: Your message of "Thu, 09 Oct 1997 22:05:13 EST." <199710100305.WAA17745@nospam.hiwaay.net>
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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
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