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From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: net.micro
Subject: Re: RS232 Voltages
Message-ID: <5818@utzoo.UUCP>
Date: Tue, 23-Jul-85 12:33:03 EDT
Article-I.D.: utzoo.5818
Posted: Tue Jul 23 12:33:03 1985
Date-Received: Tue, 23-Jul-85 12:33:03 EDT
References: <55@ssc-vax.UUCP>, <1720@ecsvax.UUCP>
Organization: U of Toronto Zoology
Lines: 19

Note that running your RS232 drivers off a +-5 supply will quite possibly
violate the RS232 standard, because +5 supply voltages (from which the -5
is probably derived) are not guaranteed to be >= 5.000V, and 5V is the
absolute minimum for the transmitter end of the RS232 spec.

Using +5 and 0 as "RS232" is a gross violation of the standard, and works
sometimes only because most RS232 receivers are very tolerant.  Put that
together with a sloppily-done receiver and you're up the creek.

Note also that the major reason you can run RS232 for far longer distances
than the spec says, is that the transmitters and receivers are overbuilt.
If you start cutting corners, you have to be very careful.  To quote (very
approximately) McNamara's book on data communications:  "In the old days,
every designer took a bash at meeting RS232 in his own way.  The result was
a lot of headaches.  Nowadays most everybody uses the standard chips [at
standard supply voltages -- hs] and the results are much better."
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry