Date: Sun, 02 Nov 1997 17:21:33 +1030 From: Mike Smith <mike@smith.net.au> To: "Jamil J. Weatherbee" <jamil@trojanhorse.ml.org> Cc: Mike Smith <mike@smith.net.au>, hackers@freebsd.org Subject: Re: 7400 gates effected by probe routine Message-ID: <199711020651.RAA00596@word.smith.net.au> In-Reply-To: Your message of "Sat, 01 Nov 1997 22:41:01 -0800." <Pine.BSF.3.96.971101205743.671B-100000@trojanhorse.ml.org>
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> > I'll be more specific, I want to be able to detect an 8255 on ISA bus. In a universal and safe fashion? AFAIK, no can do. We use a lot of 8255's, and all of my drivers do autodetect. Because we're running on a custom bus, I can use different techniques, but I've never come up with one that will let you detect an 8255 standalone; you have to know what's connected to it, or that it's there as part of some other hardware. > What bothers me is that I would really love > to have a probe routine here, but the board doesn't read anything > distinctive on startup. So I got an Idea on how to do a probe, but it > would not be safe for all situations is my problem (i.e. I don't know what > somebody might have connected to there dio board, the only safe state is > high impedance inputs, which in this case I think have pullups connected.) > If they do, I wonder if the outputs utilize open collector logic? No. Depending on the particular 8255 in question they're either NMOS (older parts) or CMOS outputs. > Unfortunately I don't have a schematic, because Industrial Computer Source > does not distribute those. It's not too hard to trace the board back, but regardless this card of yours has the change-of-state stuff; does it have a status register for same? Also, I don't have the right databook(s) here; what does the MODE/BITC register on the 8255 read after reset? > Any clues on how to safely do probing? Not really, no, unless MODE reads != 0xff. The 8255 isn't what you'd describe as the most sophisticated bit of hardware. I'd still recommend looking at the driver as having to address the hardware on the other side of the card, rather than the card itself. mike
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