Date: Thu, 09 Oct 1997 22:05:13 -0500 From: dkelly@hiwaay.net To: freebsd-hackers@FreeBSD.ORG Subject: Re: Reliable probing techiniques for isa bus? Message-ID: <199710100305.WAA17745@nospam.hiwaay.net> In-Reply-To: Message from Julian Elischer <julian@whistle.com> of "Thu, 09 Oct 1997 17:53:42 PDT." <343D7C95.52BFA1D7@whistle.com>
next in thread | previous in thread | raw e-mail | index | archive | help
> 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.
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?199710100305.WAA17745>