Date: Sat, 1 Nov 1997 22:41:01 -0800 (PST) From: "Jamil J. Weatherbee" <jamil@trojanhorse.ml.org> To: Mike Smith <mike@smith.net.au> Cc: hackers@freebsd.org Subject: Re: 7400 gates effected by probe routine Message-ID: <Pine.BSF.3.96.971101205743.671B-100000@trojanhorse.ml.org> In-Reply-To: <199711020255.NAA00542@word.smith.net.au>
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I'll be more specific, I want to be able to detect an 8255 on ISA bus. This goes back to the "dio" driver, you probably have or can get the source code from the "gnats". 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? Unfortunately I don't have a schematic, because Industrial Computer Source does not distribute those. My proposed probe routine is as follows, but I am uncertain about a lot of things in particular with the dio 48s: I have already verified it works, I am just worried it might over time destroy some equipment if the equipment is setup to give input to the dio board. There are two 8255's (@ iobase+0 & iobase+4) reset state for the 8255 is all inputs. a = inb(iobase+0) //input on port a outb (iobase+3, 0x8b) //set port a to output b = inb(iobase+0) //should be = 0 outb (iobase+3, 0x9b) //back to all inputs Well I just thought of a time this wouldn't work, if someone had equipment connected to port A all of whose inputs were 0. It is nice to see a transition but perhaps I should just switch each port to output and check for it being in Any clues on how to safely do probing? On Sun, 2 Nov 1997, Mike Smith wrote: > > > > During a device probing routine I noticed that it is possible for the > > outputs of two 7400 TTL gates one in the high state and one in the low > > state to be connected together for not more than 10 microseconds. I was > > looking at a transistor level diagram of a 7400 and would like to verify > > the following. That this should not damage the gate itself, just pull > > about 10 times the normal current. I don't see anyway to get around this > > without just removing the probe routine. > > Jamil, during the "probing routine" it is more likely that crazed mice > will be copulating on your motherboard than that two 7400 devices will > actually be present in your system, let alone have their outputs in > contention. > > Can you perhaps clarify the realities of the situation and what the > actual problem that you believe that you have is? > > mike > > > >
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