NetNews Usenet Archive 1992 #19/NN_1992_19.iso/spool/comp/dcom/modems/12442

Internet Message Format | 1992-08-23 | 2.5 KB

Path: sparky!uunet!usc!randvax!edhall
From: [e-mail] (Ed Hall)
Newsgroups: comp.dcom.modems
Subject: Re: 16550 vs. 16550AFN
Message-ID: <[e-mail]>
Date: 24 Aug 92 03:29:52 GMT
References: <[e-mail]> <[e-mail]> <[e-mail]>
Sender: [e-mail]
Organization: RAND
Lines: 38
Nntp-Posting-Host: ives.rand.org

In article <[e-mail]> [e-mail] writes:
>berger@atropa (Mike Berger) writes:
=>It's not even something you should worry about.  The concern is that
=>the modem has to keep up with the maximum data throughput rate.  If
=>you use an external modem, then your computer's hardware (your serial
=>port) has to be able to handle the data rate.  When you use an
=>internal modem, you eliminate that potential bottleneck.  Who cares
=>what kind of uart is used on an internal modem (or indeed, that one
=>is used at all)?  If the modem can handle the maximum data rate,
=>then a different uart chip will make no difference at all.  A modem
=>doesn't have to emulate a 16550AFN to have buffering on board.
>
>Excuse me? The software that is fetching the bytes doesn't act
>*at all* differently if the modem is internal. The PC *still*
>has to go thru a UART to get the data from the modem! The
>bottelneck still exists.

The "bottleneck" is the requirement--with "simple UARTs"--that the
computer handle each character within a single character-time, or else
the next will be lost.  No exceptions.  If it takes more than this time
to perform a disk read, fetch data from extended memory, or whatever,
characters get lost.  This is true even though /on average/ there is
more than enough CPU to handle the characters at the full rate.

16550's ease this problem by buffering characters, so that an interruption
of several character-times can be tolerated.  In addition, since several
characters can be transfered on a single call to the serial interrupt
routine, greater efficiency can be had.

Internal modems can do the same thing, by buffering characters just like
a 16550.  Even though it might use (or emulate) a simple UART internally
to allow for easy software compatability, the modem doesn't need to pump
characters through it when a data overrun will occur, and can buffer
them for a while in its on-board processor.  Some modems do this (Boca,
for example).  Others don't.  (I'd like to know just which ones do, too.)

        -Ed Hall
        [e-mail]