[ On Wednesday, September 29, 1999 at 00:36:56 (GMT), Christos Zoulas wrote: ]
> Subject: Re: Behavior of DTR changed between 1.3.3 and 1.4.1???
>
> In article <Pine.SOL.3.96.990928171737.12297F-100000@marcy.nas.nasa.gov>,
> Bill Studenmund <wrstuden@nas.nasa.gov> wrote:
> >On Tue, 28 Sep 1999, Grey Wolf wrote:
> >
> >> On Tue, 28 Sep 1999, John Refling wrote:
> >> 
> >> # 
> >> # >> discussion on adding a resistor on the RX line to pull voltage down
> >> # >> to -10 volts, when inactive.........  proposed
> >> 
> >> Just a minor nit:  I thought voltages for RS232 were supposed to be
> >> between 0 and 5, since RS232 originally ran on TTL.
> >
> >Nope, a bit-0 is a positive voltage, and a bit-1 is a negative voltage.
> >positive and negative voltages are from 3 to (I think) 25 volts. Also
> >don't forget that, back in the day when this was developed, logic boards
> >commonally had -5 V and some 12 V volatages around. :-)
> 
> I think that Grey Wolf is right here. RS232 was defined for TTL voltages.
> RS422 which is the most common form we see today is differential.

Nope, Bill Studenmund was right.  RS232 did arrive on the scene at about
the same time as TTL, but they didn't use the same signal levels.

Though my memory for such details is often fuzzy, my bookshelf usually
provides the necessary information (especially now that it's finally
been unpacked! ;-).

"RS-232 was first specified by the EIA in the 1960's" according to
"RS-232 Simplified" by Byron W. Putman (ISBN 0-13-783499-3, Prentice
Hall, 1987).  This is usually the first reference I turn to when it
comes to anything to do with RS-232 (though of course it's not as
authoritative as the original standards documents, which I wish I had
copies of!  ;-)

Indeed what it all boils down to is that a logic "1" value is
represented by a voltage between +3 and +25 VDC, and a logic "0" value
is represented by a voltage between -3 and -25 VDC.

In my experience most common true-to-the-standard devices use +/-12 VDC,
and Putman says that typical values are more like +/- 9 to 12 VDC.

The original IBM PC was one of the first common systems to use
non-standard line drivers (75154) operating at TTL levels.

I've encountered more than enough equipment that baulks at +/- 5 VDC
too, and of course with only 5 VDC you can't very well exceed the
standard recommended cable lengths either, never mind the other problems
this restricted signal level can introduce!  ;-)

-- 
							Greg A. Woods

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