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.email@example.com. (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. firstname.lastname@example.org
Cray Research, Inc. Virgil (612) 683-5872
655 Lone Oak Drive
Eagan, MN 55121