Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!apple!netcom!gandrews
From: gandrews@netcom.COM (Greg Andrews)
Newsgroups: comp.dcom.modems
Subject: Re: Why no PC modems without UART (re: Beware the Telebit T18pc)
Summary: PC modem without a UART is an incompatible modem. (kinda long)
Keywords: uart chip emulation modem
Message-ID: <23490@netcom.COM>
Date: 10 Feb 91 06:47:44 GMT
References: <1991Feb9.022024.10932@wsrcc.com> <23314@netcom.COM> <1991Feb9.164603.17731@europa.asd.contel.com>
Followup-To: comp.dcom.modems
Organization: Netcom - Online Communication Services  UNIX System {408 241-9760 guest}
Lines: 80

In article <1991Feb9.164603.17731@europa.asd.contel.com> 
enger@seka.scc.com writes:
>
>I believe one of (perhaps THE) main purposes of a UART is to perform
>the serial<-->parallel conversion between the (serial) data communication
>channel, and the host's (parallel, byte-wide) i/o data paths.  Is this correct?
>

Yes, it adapts a parallel computer bus to a serial RS232 'bus'
(if you're willing to consider RS232 a bus)

>
>It also my understanding that many of the fancier modems are already
>operating on byte-wide data internally.  
>

Internally?  Yes, if they have error correction and aren't merely bit pumps.

>
>Some compression algorithms use coding schemes based on reducing the bits 
>required to transmit frequently occuring symbols (characters or bytes).
>The modem must also be able to recognize x-on and x-off symbols in the 
>data stream, etc.  Thus it would seem that most modern modems MUST view
>the host's IO stream in a byte-wide (character oriented) fashion.
>

Again, yes, if the modem is using error correction at the moment.  If it
isn't, then it's just operating as a bit pump.

>
>If these statements are true, then it WOULD seem desirable to eliminate
>the UART from the data path between the modem card and the host.
>Indeed, it would be nicer still to 'widen' the data path between the 
>modem and the host.  That is, truly parallel-based internal modems
>could utilize the full width of the I/O bus available in the host.
>Still fancier optimizations are possible.  Modems could perform DMA
>transfers to memory, become bus-masters, or employ other techniques to
>lessen the per-character interrupt load on the host.
>
>While these techniques seem feasible, they all require custom drivers
>(and/or application programs) to operate the card.  This may render
>some 'classic' pc-caliber software packages unusable.  So, the marketing
>weenies will probably advise management against developing such products.
>
>Comments?

In theory, yes it would seem desirable to get rid of the UART and deal
with the data directly on a byte level.  If necessary, the modem could
attach start, stop, and/or parity bits to the characters just like the
UART chip does.

Of course, this instantly makes the modem completely incompatible with
all existing communications software written for the PC.  The reason
internal modems have a UART in the first place is so they will appear
to be a standard serial port, guaranteeing compatibility with everyone's
BIOSes, comm programs, and port drivers.

IMHO, a modem manufacturer will have enough to worry about making their
MNP4/MNP5/V.42/V.42bis and even their PEP/HST/Ping-Pong/V.32/V.22bis
(pick one) code work.  Expecting them to write SCO/Interactive/AT&T/DOS
drivers and make them work with the entire installed base is probably
asking for more software development and support than they can provide,
especially if that modem must be available at the prices demanded today.

Heavy software development and support require that the product make
more money than a UART interface modem in order to break even or reap a
profit.  Incompatibility with even part of the market (how many MS-DOS
comm programs bypass DOS/INT14/BIOS drivers and access the UART directly?)
means the modem will have lower sales.  Lower sales means the modem must
be priced higher to make money (another technique for reducing sales 
figures)...etc...etc.

The way I see it, there aren't enough advantages to make it worth anyone's
while just yet.  But don't take my word for it...perhaps we're theorizing
about a product that someone is developing in their garage right now. :-)

-- 
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| Greg Andrews      |   gandrews@netcom.COM |
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