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From: Magill@upenn.CSNET (CETS Operations Manager)
Newsgroups: mod.computers.vax
Subject: Re: high-speed term connection
Message-ID: <8605290009.AA07537@ucbvax.Berkeley.EDU>
Date: Tue, 27-May-86 17:12:00 EDT
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Posted: Tue May 27 17:12:00 1986
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The EIA RS232 length limit of 50 feet was created around a certain wide standard
of voltage levels and corresponding line losses and load factors. 
The EIA signal may be from + or - 3 to + or - 25 volts with the specification 
that when properly loaded (3000-7000 ohms) the signal at the "interface point"
shall be not less than 5 volts nor more thatn 15 volts in magnitude.

Therefore sufice it to say that if you:
a) increase receiver sensitivity (being able to "read" a low incoming voltage
   level (+- 3) while not being killed by a high level (+-15), 
b) increase your output transmitter so that you stay closer to the +- 15 volt 
   signal level than to the +- 5 level
c) use low loss transmission cable 
d) use shielded transmission cable

Use of any or all of the above techniques will markedly change the distances
available to you for an EIA RS232 connection. You will discover that terminal
"A" will work reliably at high speed (9600 -19.2k) for 300 feet, but that
terminal "B" will only work reliably at a maximum speed of 2400 baud at 300
feet, and that terminal "C" won't even work beyond 100 feet.

Today, all of the above is changing as the Communications world evolves from
the "unbalanced" world of EIA RS232 into the "unbalanced/balanced" world 
of EIA RS443 into the "balanced" world of EIA RS449. You will note that on page
4 of your latest DECDirect catalog update that DEC423 appears.

One of the major goals of the new standards is the achievement of higher
transmission speeds over longer distances, reliably.