Date   : Thu, 13 Nov 1986 22:15:21 GMT
From   : "Paul R. Grupp" <GRUPP@mit-ai.ARPA>
Subject: N* Advantage = Dumb terminal?

    Date: Mon, 10 Nov 86  8:32:26 EST
    From: Robert Bloom AMSTE-TEI 3775 <rbloom at APG-1.ARPA>

    I'm trying to use my NorthStar Advantage (a all-in-one computer) 
    as a dumb terminal to a TurboDOS system.  The problem is that 
    every commo package I've tried so far is too slow for any speeds 
    higher then 1200 baud or so.  I want 4800 baud minimum, 9600 baud 
    desired.  (19.2k? dream mode on)
...
    The effect is characters are lost, most commonly one or two after
    a linefeed.
...
This is a common problem with any memory mapped display that is updated
by the host CPU.  The problem here is that any line feed that requires
the screen to scroll (e.g. <LF> on the last line) will need the CPU
to move EVERY character on the screen up one line then clear the last
line.  This takes a great deal of CPU time and while this is going on
the CPU can't poll the port to check for new incomming data.  Here are
three solutions that I've used in order of ease to implement.

 1.  Have the host send PADD characters (or pause time) after it sends
     a <LF>.  (note clear screen takes a while too)
 2.  Write interupt drivers for the COMM port that fills a buffer that
     the term program goes to when looking for input.
 3.  Write your own screen driver that looks at the comm port after
     each single line is moved, and buffers any waiting data between
     it's moves.
 (4) Get a terminal or new system! 8^)

Using any of 1-3 I've gotten an AVERAGE throughput of about 80K baud!

-Paul