Date: Thu, 18 Sep 1997 22:18:39 +0200 From: j@uriah.heep.sax.de (J Wunsch) To: hackers@FreeBSD.ORG Subject: Re: INB question Message-ID: <19970918221839.VL10449@uriah.heep.sax.de> In-Reply-To: <199709181039.UAA00245@word.smith.net.au>; from Mike Smith on Sep 18, 1997 20:09:55 %2B0930 References: <199709180728.AAA19024@usr06.primenet.com> <199709181039.UAA00245@word.smith.net.au>
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As Mike Smith wrote: > Note that Joerg's comment about a nonresponding bus giving random > values is *WRONG* for most busses; certainly at least ISA and PCI. > > The ISA specification explicitly requires bus pullup resistors. It may > be unwise to depend on reading 0xff back-to-back with a previous read/ > write operation, ... That's why i wrote ``unspecified, with a tendency to 0xff''. > but the reader is welcome to calculate the RC time constant for a > transmission line with a few pF of capacitance and a 10K (or less) > pullup. j@uriah 132% perl -e 'print 50e-12 * 10e3; print "\n"' 5e-07 I think 50 pF is rather an understatement. 0.5 µs doesn't seem to be terribly short, but IIRC, ISA inb's are artificially deferred by 1.25 µs, so chances are good to actually see 0xff. I wouldn't rely on it for a back-to-back read, however. -- cheers, J"org joerg_wunsch@uriah.heep.sax.de -- http://www.sax.de/~joerg/ -- NIC: JW11-RIPE Never trust an operating system you don't have sources for. ;-)
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