From owner-freebsd-hackers Thu Sep 18 13:25:53 1997 Return-Path: Received: (from root@localhost) by hub.freebsd.org (8.8.7/8.8.7) id NAA03781 for hackers-outgoing; Thu, 18 Sep 1997 13:25:53 -0700 (PDT) Received: from sax.sax.de (sax.sax.de [193.175.26.33]) by hub.freebsd.org (8.8.7/8.8.7) with SMTP id NAA03773 for ; Thu, 18 Sep 1997 13:25:48 -0700 (PDT) Received: (from uucp@localhost) by sax.sax.de (8.6.12/8.6.12-s1) with UUCP id WAA14515 for hackers@FreeBSD.ORG; Thu, 18 Sep 1997 22:25:40 +0200 Received: (from j@localhost) by uriah.heep.sax.de (8.8.7/8.8.5) id WAA08304; Thu, 18 Sep 1997 22:18:39 +0200 (MET DST) Message-ID: <19970918221839.VL10449@uriah.heep.sax.de> 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 References: <199709180728.AAA19024@usr06.primenet.com> <199709181039.UAA00245@word.smith.net.au> X-Mailer: Mutt 0.60_p2-3,5,8-9 Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8bit X-Phone: +49-351-2012 669 X-PGP-Fingerprint: DC 47 E6 E4 FF A6 E9 8F 93 21 E0 7D F9 12 D6 4E Reply-To: joerg_wunsch@uriah.heep.sax.de (Joerg Wunsch) In-Reply-To: <199709181039.UAA00245@word.smith.net.au>; from Mike Smith on Sep 18, 1997 20:09:55 +0930 Sender: owner-freebsd-hackers@FreeBSD.ORG X-Loop: FreeBSD.org Precedence: bulk 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. ;-)