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From: mcdonald@aries.scs.uiuc.edu (Doug McDonald)
Newsgroups: comp.sys.ibm.pc.misc,comp.dcom.modems
Subject: Re: 4.77MHz PC sufficient for 19200 bps modem?
Message-ID: <1991Jun19.225328.20629@ux1.cso.uiuc.edu>
Date: 19 Jun 91 22:53:28 GMT
References: <1155@isgtec.UUCP> <1991Jun18.201125.6198@cs.cmu.edu>
Sender: usenet@ux1.cso.uiuc.edu (News)
Organization: University of Illinois at Urbana
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In article <1155@isgtec.UUCP> ted@isgtec.uucp (Ted Richards) writes:
>In article <1991Jun18.201125.6198@cs.cmu.edu> jch+@cs.cmu.edu (Jonathan Hardwick) writes:
>> Question for the net:
>> 
>> A housemate has an original Big Blue IBM PC, running at the blazing
>> speed of 4.77MHz.  We want to use this as a dumb terminal to access
>> our university workstations.  The university offers 2400 bps service
>> with MNP level 5, and a 9600 bps or 19200 bps MCN service using DOV
>> modems.  We'd really like the extra speed if possible.  Is the PC
>> going to be able to keep up with 19200 bps?  How about 9600 bps? 
>
>I have some bad news for you.  I have one of these geriatric beasts,
>too, and it doesn't even come close to keeping up at 9600 baud.  The
>problem is in the screen updates, not the comms port, which manages file
>transfers at almost full speed (~ 860-920 cps).
>
>The speed varies depending on the comms programm.  I have tried Procomm,
>Telix and two varieties of MS_kermit.  Telix is really awful
>(unfortunate, because I like the programm a lot).  Even with Telix doing
>its own screen updates (the fastest mode), it fills the screen at
>perhaps 300 cps when scrolling, somewhat better when clearing the screen
>first.  The latest Kermit is not much better.  Procomm was, I think, a
>little faster, but I tried it a year ago, so I don't remember exactly.
>
>The best performance, perhaps 500-600 cps, comes from a very old copy of
>Kermit, dated 1982 or 1983 that I found lying around on an old diskette.
>
>Perhaps someone else has found another comms program that can keep up?
>
>By the way, I am using a Telbit T1000 on my end, talking to a T2500 (at
>9600 baud) at work.
>
>> Oh, and if the PC *can't* keep up, what sort of motherboard upgrade
>> would we need?  10 MHz 8086?  80286?
>
>Can't help you here, since I haven't done anything about it (I'm saving
>up for a 386-25, myself).
>
>--

A plain PC at 4.77 MHz can EASILY keep up with 19200 baud including output
to the screen. You just have to have an EGA card or higher (or maybe
a CGA or mono, I've never tried). You do it by scrolling NOT by actually
moving characters around in video memory, but by setting up a double-length
video buffer (twice the length of the screen) and keeping TWO copies of
your text in it in double-circular-buffer mode. You then just move the
start-of-memory pointer on the video card to scroll. 


And, amazingly enough, on my 386 this works fine up to 9600 baud
**in graphics mode**!!!!


Doug McDonald