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Path: euryale.cc.adfa.oz.au!newshost.anu.edu.au!harbinger.cc.monash.edu.au!news.rmit.EDU.AU!news.unimelb.EDU.AU!munnari.OZ.AU!news.ecn.uoknor.edu!news.wildstar.net!news.sdsmt.edu!news.mid.net!newsfeeder.gi.net!newsfeed.internetmci.com!tank.news.pipex.net!pipex!oleane!jussieu.fr!rain.fr!news.sprintlink.net!news-dc-9.sprintlink.net!netnews.nwnet.net!symiserver2.symantec.com!usenet From: tedm@agora.rdrop.com Newsgroups: comp.unix.bsd.freebsd.misc Subject: Re: Modem at COM3: can't find it! Date: 27 Jun 1996 05:23:19 GMT Organization: Symantec Corporation Lines: 30 Message-ID: <4qt5s7$dkd@symiserver2.symantec.com> References: <4q4bho$7ht@netnews.upenn.edu> <4q6227$qc0@panix.com> <31CE3919.4A982BB@lambert.org> <4qqeef$cj@anorak.coverform.lan> Reply-To: tedm%toybox@agora.rdrop.com NNTP-Posting-Host: 198.6.34.3 X-Newsreader: IBM NewsReader/2 v1.2 In <4qqeef$cj@anorak.coverform.lan>, brian@awfulhak.demon.co.uk (Brian Somers) writes: >Terry Lambert (terry@lambert.org) wrote: >: J Wunsch wrote: >: ] Not that i really like to defend M$, but that's rather the fault of >: ] Little Blue's geniusses of engineers. They've been to stingy to use a >: ] tri-state gate for driving the IRQ line, which would have allowed for >: ] shared interrupts, but yet assigned the default IRQs for com1/com3 and >: ] com2/com4 twice. > >: Actually, there is no IBM standard for com3/com4. This was done >: by a modem manufacturer who needed to plug his internal modems [some deleted] >Are you sure ? I thought IBM made an "ask the BIOS" standard. I havn't >got the book handy for the next couple of weeks - I can't even remember The IBM BIOS did specify an IO port for com3 and com4, but in all the PC BIOS'es I ever heard of the COM port I/O routines were polled mode only, no interrupts. They were intended to be used with serial printers that would not be sending back data, thus polling the UART was considered acceptable. What the modem manufacturer did was attempt to associate IRQ lines with the IBM-specified ports that were already in use. In any case, it wouldn't have mattered if IBM had tri-stated the IRQ lines, since some el-cheapo manufacturer would have undoubtedly have made an add-in card that didn't tri state, and the rest of them would have probably rapidly followed suit. After all, from a technical standpoint, the IBM PC/XT was considered rather lame compared to the other computers available of the day.