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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: Little help required........
References: <email@example.com> <firstname.lastname@example.org> <3DAED542.email@example.com> <3DB005D1.firstname.lastname@example.org>
Date: Fri, 18 Oct 2002 13:24:41 GMT
NNTP-Posting-Date: Fri, 18 Oct 2002 06:24:41 PDT
Fred Bloggs wrote:
> Jim Thompson wrote:
> > |
> > Fred, Been drinking already this morning ?:-)
> > Where did you dream up this "three-state" stuff?
> > ...Jim Thompson
> Okay- I checked what little of the now ANSI/TIA/EIA-RS232 I could find.
> This thing is up to version F and has been extended to lower voltage and
> 512Kbps in succeeding standards. It is strictly a logical two-state
> unless you want to read something into the "fail safe" paragraph which
> specifies four failure conditions that require detection, and then it
> does become a three-state in a way. I am interested in knowing why this
> interface was ever bi-polar given that it's a two state and the receiver
> threshold is allowed to float around inside a +/-3V window. Usually
> bi-polar operation allowed for magnetic coupling to eliminate common
> mode interference, but you can't do that with RS-232,or can you.
I suppose the bipolar standard may have roots in the older TTY
standards. 65ma and 20ma current loop TTY suffered from a sort of
distortion which may have been called bias distortion (it's been a good
many years since I did TTY loops and memory is dim). The distortion
caused marking to get shorter as the distance from the transmitter
increased. The distortion becomes symmetric when voltage is sent instead
of current if the threshold is set at one half of the sending end
marking voltage. However, if the sending end marking voltage has a wide
specification or line loss can reduce amplitude so that the effective
marking voltage is not known, bipolar voltage signals become
interesting. If the sending end marking voltage and spacing voltage are
the same but opposite sign, the receiving end does not need to know the
voltages since distortion free decisions can be made around zero volts.
I don't know if this is the reason or not but it is certainly an
advantage of a bipolar signal in the type of service in which RS-232
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com
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