Date   : Fri, 30 Oct 1992 17:41:23 GMT
From   : sun-barr!cs.utexas.edu!zaphod.mps.ohio-state.edu!usc!sol.ctr.columbia.edu!The-Star.honeywell.com!umn.edu!noc.msc.net!uc.msc.edu!shamash!timbuk.cray.com!hemlock.cray.com!jot@ames.arc. (Otto Tennant)
Subject: Re: 5v out serial port

The RS-232 standard, defines a "mark" as a voltage between +3V and
+12V; a "space" as a voltage between -12V and -3V (or, just possibly,
the other way around.)  (And just maybe 15V, not 12V?)  Notice that 0V
doesn't "mean" anything.

However, folks noticed that 0V and +5V would "operate" many modems,
even though it shouldn't.

If you are seeing 0V and +5V, you are probably trying to plug directly
into the UART, which is maybe not the best thing in the world to do;
it was intended that there be some sort of line driver between the
UART and the rest of the world.  I would not care to guess what
happens to one of those chips if it is connected to a modem which
applies -12V to the input.
--
==============================================================================
J.Otto Tennant      Forsan et haec olim meminisse juvabit.         jot@cray.com
Cray Research, Inc.              Virgil                          (612) 683-5872
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