Date: Wed, 30 Jul 1997 03:55:20 GMT From: mouth@ibm.net (John Kelly) To: Michael Smith <msmith@atrad.adelaide.edu.au> Cc: msmith@atrad.adelaide.edu.au, freebsd@atipa.com, hackers@FreeBSD.ORG, terry@lambert.org Subject: Re: Sharing interrupts Message-ID: <33e1a2da.17148628@smtp-gw01.ny.us.ibm.net> In-Reply-To: <199707292329.IAA15471@genesis.atrad.adelaide.edu.au> References: <199707292329.IAA15471@genesis.atrad.adelaide.edu.au>
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On Wed, 30 Jul 1997 08:59:50 +0930 (CST), Michael Smith wrote: >John Kelly stands accused of saying: >> >More specifically, you can only allow _one_ device to drive an ISA >> >IRQ line at any given time. >> >> Not strictly true, see below. > >That depends on whether you feel like violating the bus standard. It can be done without violating any standards. In fact, the TTL logic level standards are what make it possible. With your being knowledgeable in "bus interface design," it's strange you don't already know this. >> >cross-reference the 8250 datasheet for an understanding of the quirks >> >and complexities involved in getting this right. It is _not_ trivial. >> >> I have. It's not hard at all. > >It sounds suspiciously like you are either Marilyn Vos Savant, or >underestimating the situation. It's sounds like you're jumping to conclusions. You haven't read my explanation of the technique yet. >Very few boards these days use the 8259. Most, if not all, of the >current chipsets use a logically equivalent macrocell, however their >electrical characteristics are likely to be very different to those of >a 70's vintage NMOS device. You're the one who brought up the 8259, calling it an "8250." It established a reference design, and its input logic level specs agree precisely with the TTL logic level standards. Even if clone makers didn't follow the 8259's input logic voltages, they would certainly follow the input logic voltages of the TTL standard itself, which not by coincidence, match the input logic voltages of the 8259. >Using diodes and resistors to convert totem-pole outputs into >open-collector-like outputs is highschool electronics. What a negative spin on a positive idea. Why should it require an electrical engineer to make a few simple calculations and modifications? Your smearing phraseology is not very persuasive. >You will note my employer's field in my .sig; my work profile covers >bus and interface design as well as OS and UI development. I do note this: some people swagger about like intellectual bullies, wearing a resume like a chip on their shoulder. >You're quite welcome to butcher your cards and live with the potential >uncertainties involved in what you're doing Butcher my cards? Now you're losing credibility. Anyone with real experience in electronics should be able to solder a few simple parts to a PCB without ruining the board or its components. Or haven't you heard of the common practice of rework and repair? If you can offer some genuine criticism, give it another try. But first read the technique which I will post in a day or so, and attack it on a factual basis, if you can. >The problem arises when you claim that this is a universal case: I never said it was a "universal case." You're twisting my words. First you said any high schooler can do it, and now you're implying it won't work. Well, make up your mind, which is it? >your statements are not helpful when it comes to getting >the message across. >]] Mike Smith, Software Engineer msmith@gsoft.com.au >]] Genesis Software genesis@gsoft.com.au Exactly what I would say about your smear tactics. The notion that serial cards with totem-pole outputs cannot share an edge-triggered ISA interrupt is a widespread misconception, promoted and sustained by those who believe that "tampering" with a factory product amounts to technologic sacrilege. Those of us unconstrained by such feelings can discover what lies beyond the hallowed walls of corporate conformity. Stay tuned. I'll post the technique as soon as I can write it up. John
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