Path: utzoo!utgpu!watserv1!maytag!xenitec!zswamp!root
From: root@zswamp.fidonet.org (Geoffrey Welsh)
Newsgroups: comp.dcom.modems
Subject: Questions on High speed UARTS
Message-ID: <4889.274A0A5B@zswamp.fidonet.org>
Date: Tue, 20 Nov 90 20:54:41 EDT
Organization: Izot's Swamp BBS - Kitchener, Ontario

Robert Nee (nee@cf_su14.Salomon.Com ) wrote:

 >1       I read in PC Magazine that in order to use a high speed
 >        modem at peak efficiency you must have the 16550 UART
 >        chip installed in your computer. (not an 8250 or 16450)

   The 16550 simply offers a 16-byte FIFO buffer in stead of a transmit hold 
register and receive data register. This can be importent if (a) you're using 
an XT which can't process the data quickly enough, and which loses bytes 
occasionally because of it (the 16550 stores up to 16 bytes, to be fetched 
whenever the XT gets around to it); this usually happens at 19,200 or 38,400 
bps. (b) you're using extended memory under MS-DOS on a 286; interrupts may be 
lost when a 286 switches from protected mode to real mode and the 16550 
prevents loss of characters along with it. (c) You're using a Perstor ARLL 
controller, whose long DMA cycles can prevent the CPU from fetching a byte 
from the UART before the next arrives.

   I'm sure there are more situations in which a 16550 would be helpful, but 
these are the main ones I've encountered.

   The bottom line is that the 16550 is ideal for making sure that your system 
isn't losing characters. If your system already isn't, then you will never 
notice the difference between a 16550 and whatever you replaced with it.

   I always have a hard time convincing people that throughput won't improve 
with a 16550. What it boils down to is this: the 16550 helps your CPU keep up 
with the data. If your CPU isn't keeping up with the data right now, you'd 
KNOW it.
 

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