Date: Sun, 2 Nov 1997 19:02:16 -0800 (PST) From: "Jamil J. Weatherbee" <jamil@trojanhorse.ml.org> To: Mike Smith <mike@smith.net.au> Cc: =?iso-8859-1?Q?=DEor=F0ur?= Ivarsson <totii@est.is>, hackers@FreeBSD.ORG Subject: Re: 7400 gates effected by probe routine Message-ID: <Pine.BSF.3.96.971102185606.1288A-100000@trojanhorse.ml.org> In-Reply-To: <199711030034.LAA00455@word.smith.net.au>
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There was one other thing with this board that I have thought about repeatedly. It has a jumper for tristated outputs, which would allow safe probing. However the way the tristate system works, it is either enabled or disabled for all outputs so when changing the mode of a port the outputs momentarily toggle to high impedance inputs, which defeats the purpose of having seperately controllable /dev/dio0? nodes. All in all I think it is just bad board design, nonetheless I think that anyone using one of these probably knows how it is set and doesn't need a probing routine. In other words the damn tristate bit shouldn't of been mapped over On Mon, 3 Nov 1997, Mike Smith wrote: > > I am not sure at the moment but I have been working with Rocwell 6522 > > PIA and I think it should be possible to probe 8255 in same manner as > > 6522, send byte to output ports and read back, if they are defined as > > output you should get the same back out from them. I think you can > > also with the control register. > > a) The 8255 defaults to all-ports-input at reset. > b) It is unacceptable to re-mode the 8255 and then write random garbage > into its output registers, as this may have undesirable or fatal > effects on attached hardware. > > > Do you have the 8255 datasheet ? it is available from Intel website > > I have (now) in front of me the Intel Peripheral Components databook. > This confirms that it is not possible to probe the 8255 in any useful > fashion without knowing what is connected on the other side. > > mike > > >
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