From owner-freebsd-hackers Thu Oct 9 20:05:24 1997 Return-Path: Received: (from root@localhost) by hub.freebsd.org (8.8.7/8.8.7) id UAA18064 for hackers-outgoing; Thu, 9 Oct 1997 20:05:24 -0700 (PDT) (envelope-from owner-freebsd-hackers) Received: from fly.HiWAAY.net (root@fly.HiWAAY.net [208.147.154.56]) by hub.freebsd.org (8.8.7/8.8.7) with ESMTP id UAA18059 for ; Thu, 9 Oct 1997 20:05:19 -0700 (PDT) (envelope-from dkelly@nospam.hiwaay.net) Received: from nospam.hiwaay.net (tnt1-16.HiWAAY.net [208.147.147.16]) by fly.HiWAAY.net (8.8.7/8.8.6) with ESMTP id WAA20789 for ; Thu, 9 Oct 1997 22:05:16 -0500 (CDT) Received: from nospam.hiwaay.net (localhost [127.0.0.1]) by nospam.hiwaay.net (8.8.7/8.8.4) with ESMTP id WAA17745 for ; Thu, 9 Oct 1997 22:05:14 -0500 (CDT) Message-Id: <199710100305.WAA17745@nospam.hiwaay.net> X-Mailer: exmh version 2.0zeta 7/24/97 To: freebsd-hackers@FreeBSD.ORG From: dkelly@hiwaay.net Subject: Re: Reliable probing techiniques for isa bus? In-reply-to: Message from Julian Elischer of "Thu, 09 Oct 1997 17:53:42 PDT." <343D7C95.52BFA1D7@whistle.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Date: Thu, 09 Oct 1997 22:05:13 -0500 Sender: owner-freebsd-hackers@FreeBSD.ORG X-Loop: FreeBSD.org Precedence: bulk > mdean wrote: > > > > If I have an isa card which occupies 16 ports but only uses 9 of them, I've > > put in some debugging code to see what can be read from the card at boot > > time. The read/write ports are random depending on what the card in > > connected to, the control ports are write only, and so I am only left with > > the other 5 ports on the card that aren't used and the pattern they are in. > > Is it expected that you will always read 0xff from an unused port, because > > that is what I am getting? > > welcome to the pleasures of ISA > > 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. One way to tell the difference between a Genuine Apple II and clone Franklin Ace was to read an unconnected address. An Apple II would still have the last 8-bit value shifted out the video floating in the bus capacitance. There were those who used this to sync code to video. -- David Kelly N4HHE, dkelly@hiwaay.net ===================================================================== The human mind ordinarily operates at only ten percent of its capacity -- the rest is overhead for the operating system.