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Xref: sserve comp.os.linux.development:18502 comp.os.linux.misc:28620 comp.os.386bsd.questions:14176 Path: sserve!newshost.anu.edu.au!harbinger.cc.monash.edu.au!bunyip.cc.uq.oz.au!munnari.oz.au!hpg30a.csc.cuhk.hk!news.hk.net!howland.reston.ans.net!agate!barrnet.net!rahul.net!a2i!satlink!laser!jorge From: jorge@laser.satlink.net (Jorge Cwik) Newsgroups: comp.os.linux.development,comp.os.linux.misc,comp.os.386bsd.questions,comp.os.38 Subject: Re: 16550 detection Date: Wed, 02 Nov 94 02:26:30 -0400 Message-ID: <Vy65uc1w165w@laser.satlink.net> References: <TYTSO.94Oct31162517@dcl.mit.edu> Organization: FX Uucico - FX UUCP Lines: 35 tytso@athena.mit.edu (Theodore Y. Ts'o) writes: > What probably happened is those systems have cheasy internal modems that > claim to be "16550 compatible", which is a pure marketing lie. > Typically, the marketing literature will also babble about 1024 byte > FIFO's; that's the key tipoff. > > What's happening is that these modems, instead of using an authentic > National Semicondutor part, or a more quality designed clone, is using a > cheasy UART ... [...] > ... However, if you > must get an internal modem, please please please do yourself a favor and > get one which uses a genuine National Semiconductor UART, and not one of > the cheasy imitations. Why would you want an UART at all on an internal modem ? The data is serialized ... just to be deserialized again a few traces ahead in the same board. Obviouslly a waste. Most new internal modems don't have an UART, neither a 'genuine' part neither a 'cheasy' imitation. They have a 'parallel' interface that only _looks_ like an UART from the CPU point of view. Yes, some of them emulate only the 16450 registers. And some claim to have a 1K buffer, which is nothing else that the RAM buffer available in the modem. Exactly the same as the one in an external model, only that the internal flow control avoids any kind of overrun errors. But these days, most of them fully emulate a 550A for better compatiblity. This is the case of the Rockwell chipset, which is used in most "low end" modems. I don't see any reason why you would prefer a 'real' 16550A part. Jorge