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Article #151 (419 is last):
Newsgroups: freenet.sci.comp.atari.questions
From: ac937
Subject: TTL COMPATABLE
Date: Sun Jul 30 09:32:33 1989


WHAT DOES IT MEAN TTL.COMPATABLE. AND IS THE 8-BIT'S SIO PORT
TTL COMPATABLE. I HAVE A DIAGRAM OF A TTL TO RS-232 INTERFACE
I WOULD LIKE TO BUILD IF POSSABLE. ALL I GET WHEN I CALL ATARI
IS THEY WELL CALL AND YET THEY HAVN'T. ANY HELP JOHN WILL BE
GREAT FULL.
                            AC-937
                            RICH

** Answered by JOHN SUCHY (aa271) on Sun Oct 25 03:35:40 1987 **

        TTL is a particular family of
computer chips whose major feature is 
the logical definition of "low" or
"false" as between 0 and .4 volts, and
of "high" or "true" as between 2.5 and
5.0 volts.  "TTL compatible" means that
-- in your case -- the part of the 
circuit that attaches directly to the
computer shares these definitions.
        RS-232 defines high as 10 to 12
volts, and low as -10 to -12 volts.  
The TTL to RS-232 adapter basically
converts voltage levels from one
standard to the other.
        The Atati SIO port does use the TTL definitions of high and low.  One 
word of caution, however: you must make
some provision for selecting the Serial
port as opposed to any other peripheral
or you will be sending all disk drive
and printer accesses out the port.  I
bought a product called "R-Verter" a    few years ago that was supposed to work
this way, but never could get it to 
function.
        Good luck and let us know if
you get it going.

                        Your Co-sysop
                        John E. Suchy
aa271


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