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From: hull@hao.UUCP (Howard Hull)
Newsgroups: net.micro
Subject: Re: RS232 Voltages
Message-ID: <1658@hao.UUCP>
Date: Thu, 25-Jul-85 23:00:40 EDT
Article-I.D.: hao.1658
Posted: Thu Jul 25 23:00:40 1985
Date-Received: Sun, 28-Jul-85 07:51:36 EDT
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Ken Finney writes:
> The official voltages levels are +12 and -12, giving an awful lot of noise
> margin.  To save money, microcomputers years ago started using +5 and 0.
> I have seen a lot of peripherals that clip the incoming voltages to +4.7
> and 0.7 using a zener and a resistor.  These peripherals work whether you
> are using the official voltages or not.
> 
Sometimes, however, they don't work.  One way to get one that doesn't work is
to use a long, shielded twisted pair cable.  The high capacity of the cable
causes the normally sharp bit transitions to be spread out as exponentials.
Then, depending on the bit pattern, it is possible to get a shift in the bit
transition detection point if the threshold point is not exactly between the
+ and - voltage of the sending source.  This results in some bits coming out
narrower than others and, in addition to confusing the logic that locates the
bit center, this can result in either a missed bit or a misread bit.  We have
actually had this happen here with PE Fox terminals that have a threshold
separated by a clamp diode and a base-emitter junction from ground.
								Howard Hull
        {ucbvax!hplabs | allegra!nbires | harpo!seismo } !hao!hull