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From: Fred Bloggs
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Subject: Re: Little help required........
References: <email@example.com> <firstname.lastname@example.org> <3DAED542.email@example.com> <3DB005D1.firstname.lastname@example.org> <3DB00BB8.D63D1BFE@webaccess.net>
Date: Sat, 19 Oct 2002 04:09:51 GMT
NNTP-Posting-Date: Fri, 18 Oct 2002 21:09:51 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
Chuck Simmons wrote:
> 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
That's a good point, but I wonder if the persistence of this method runs
deeper. Would it make sense that bipolar signaling with very long term
DC average of zero would mitigate long term galvanic corrosion of
dissimilar metallic junctions , not only in the interface connectors
themselves but also in the building wiring between remote equipment
which will circulate the inevitable return currents due to unbalanced lines?
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