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From: poffen@sj.ate.slb.com (Russ Poffenberger)
Newsgroups: comp.dcom.modems
Subject: Re: Questions on High speed UARTS
Message-ID: <1990Nov21.191157.11004@sj.ate.slb.com>
Date: 21 Nov 90 19:11:57 GMT
References: <4889.274A0A5B@zswamp.fidonet.org>
Reply-To: poffen@sj.ate.slb.com (Russ Poffenberger)
Organization: Schlumberger Technologies, San Jose, CA.
Lines: 40

In article <4889.274A0A5B@zswamp.fidonet.org> root@zswamp.fidonet.org (Geoffrey Welsh) writes:
>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.
> 
>

Also note that I heard that these "features" of the 16550 are TURNED OFF by
default at power on, and must be specifically enabled. If you don't have a
driver or comm program that can do this, you will not notice any benefit.

Russ Poffenberger               DOMAIN: poffen@sj.ate.slb.com
Schlumberger Technologies       UUCP:   {uunet,decwrl,amdahl}!sjsca4!poffen
1601 Technology Drive		CIS:	72401,276
San Jose, Ca. 95110             (408)437-5254