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From: gemini@homxb.UUCP (Rick Richardson)
Newsgroups: net.periphs,net.micro,net.wanted
Subject: Re: update on **real** 19200 CRT
Message-ID: <1828@homxb.UUCP>
Date: Tue, 29-Jul-86 13:31:10 EDT
Article-I.D.: homxb.1828
Posted: Tue Jul 29 13:31:10 1986
Date-Received: Thu, 31-Jul-86 02:02:05 EDT
References: <671@mordred.purdue.UUCP> <71@winfree.UUCP>, <2349@cbosgd.UUCP>
Organization: PC Research, Inc.
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Xref: mnetor net.periphs:481 net.micro:5709 net.wanted:3551

> Before you decide to do this, I suggest you buy one, get your software
> running, and see if you like it.  We have lots of users here using AT&T
> 6300's as terminals.  (The 6300 has an 8MHz 8086 which will do at least
> as well as what you describe.)  The terminal emulation programs don't come
> anywhere near 19200 bps; typical throughput is more likd 2400 bps or so.
> (I can't remember the exact figures, it might have been as high as 4000
> bps.  But it was nowhere near 19200.  And it does vary from program to
> program.)
> 
> The 6300 display board does snow, just like the official IBM color card,
> so if you can arrange to get a non-snowing card, and have your program
> know about it, you might speed things up a bit.

19.2 Kbs ought to be easily achievable in a standalone situation as
the original poster suggested, even on a stock IBM PC running at 4.77Mhz.
The key is to use a truly dual ported video card, so you won't have to
wait for retrace.  At 1920 cps coming in from the 8250 you have
about 500 usec to handle each interrupt.  That's about 125
"average" instructions on a stock PC, or 250 on a fast clone.
Now, reading the 8250 and putting the character in the video
RAM is maybe a handfull of instructions.  The tricky part is
display scroll, which can happen in one of two ways: software
or hardware.  The software approach (used by IBM in the BIOS)
requires a block move of 4000 bytes which takes about 4 msec.
This is much too long if a series of new lines come along.
But, you can scroll the display by simply moving the base
address in the 6845 CRT controller with just a few instructions.
The second home brew terminal I built used a BELLMAC-8 uP and 6845
CRT controller and scrolled the screen in this exact manner.
> 
> The other problem is that the typical PC comes with a horribly braindamaged
> keyboard.  Most terminals come with reasonable keyboards, except the
> VT200 series and clones.  You can get a Keytronics keyboard, but you
> may not like the feel, and they will add $200 to your cost for the 5151.
> (If you can find a 5150 and like it, it may cost considerably less.)
> 
> 	Mark
> 
This one is easily fixed on any keyboard with removable tops.  Just
move the key (like Escape on an AT keyboard) to where it belongs,
and adjust the keybaord lookup table accordingly.  I've done it
on my keyboard.  It works.  Most of the PC clones come with "AT"
style keyboards, so you just have to exchange the Esc, tilda, and
backspace keys to get a decent arrangement.  Keyboard feel is a
matter of personal taste.  You may have to shop around to find
the feel you like.  I personally would opt for a genuine IBM AT
keybaord.

Conclusion: you CAN turn a $600 PC clone into a terminal capable
of at least 9600 baud without flow control, and probably the
full 19200, if you are willing to write tuned assembler at the
raw metal level.

Rick Richardson, PC Research, Inc. (201) 922-1134, (201) 834-1378 @ AT&T-CP
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