Path: utzoo!utgpu!watserv1!maytag!xenitec!zswamp!root
From: root@zswamp.uucp (Geoffrey Welsh)
Newsgroups: comp.dcom.modems
Subject: 16550A UART, Needed?
Message-ID: <83.283BB3AF@zswamp.uucp>
Date: Thu, 23 May 91 01:35:15 EDT
Organization: Izot's Swamp BBS (FidoNet), Kitchener, Ontario

In a letter to All, Jon Noring (noring@netcom.COM ) wrote:

 >The main question:  Is it really worthwhile replacing my 
 >current 8250 UART with a 16550A when I get a 9600+ modem,
 >or are the gains minimal?

   If your system works fine with your 8250, then you don't need a 16550A.  
Although the 16550 can reduce the CPU overhead of interrupt service, the real 
effect is, as you say, minimal.

   However, there are many systems which won't work reliably -or at all- at 
9600+ bps, and a 16550A can usually cure the problem completely.  Some things 
which *may* cause problems with high speed serial I/O (problems which will be 
solved with the use of a 16550A):

 - CPU simply too slow for baud rate (especially affects 4.77 MHz XTs)
 - 286 changing from protected to real mode (to access extended memory from 
DOS, or returning to OS/2 or QNX from a DOS compatibility box)
 - Perstor ARLL controllers (or other devices with long DMA sessions)
 - Multitasking

   None of these spells sure death for high speed serial I/O; if one or more 
applies to you, you may be able to get away with your 8250... but you're in a 
'high risk group'.

 >(of course, I'll have to make sure that my communications
 >software fully takes advantage of the 16550A UART).

   Any 8250 driver that is extremely well-written (i.e. takes into account 
the possibility that a character might arrive while the routine fetches the 
one that caused the interrupt, and therefore should also be fetched 
immediately) will work with a 16550A with buffers enabled.  Unless your 
software re-initializes the 8250 (Telix does, but it's 16550-aware), you'll be 
able to enable the buffers beforehand using a program such as FIFO.COM and 
reap the benfits.
 

--  
Geoffrey Welsh - Operator, Izot's Swamp BBS (FidoNet 1:221/171)
root@zswamp.uucp or ..uunet!watmath!xenitec!zswamp!root
602-66 Mooregate Crescent, Kitchener, ON, N2M 5E6 Canada (519)741-9553
"He who claims to know everything can't possibly know much" -me