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Path: utzoo!watmath!clyde!floyd!harpo!ihnp4!fortune!rpw3
From: rpw3@fortune.UUCP
Newsgroups: net.unix
Subject: Re: Transmitting BREAK - (nf)
Message-ID: <3034@fortune.UUCP>
Date: Wed, 11-Apr-84 07:06:31 EST
Article-I.D.: fortune.3034
Posted: Wed Apr 11 07:06:31 1984
Date-Received: Thu, 12-Apr-84 05:14:18 EST
Sender: notes@fortune.UUCP
Organization: Fortune Systems, Redwood City, CA
Lines: 38

#R:sri-arpa:-1674800:fortune:26900042:000:1559
fortune!rpw3    Apr 11 03:06:00 1984

+--------------------
| I forgot about the inversion.  A NULL is correct.  However,
| the idle state of a RS232 line is MARK = ONE = -V.
|                                                ^^
| The rs232 interface spec defines a -V as a binary state of 1 or MARK.
| 
| 				Paul McFerrin
+--------------------

Wow, is my face red! (*BLUSH*)

In my rush to correct Mr. McFerrin and in my enthusiasm for a bit of
history, I goofed (badly). He is, of course, correct. The idle state for
ALL signal lines, data and control, on an RS-232 connection is "-V"
(= ONE = MARK = "stop-bit").  "+V" is actually SPACE = ZERO = "start-bit".

[When one is working with the TTL levels corresponding to an RS-232 interface,
there is generally a layer of inverters involved, so on the TTL side, the data
is MARK = ONE = +TTL and SPACE = ZERO = ~GND. If you look at the pretty little
picture "oliveb!jerry" drew, you will see a start-bit going LOW, just like
I'm used to watching it on a 'scope. On the other hand, the control signals
are usually low-asserted on the TTL side, so I have no excuse.]

Anyway... the point of the story was to give some background/insight as to the
reason the data lines use "-V" to mean "one", not "zero", just the opposite of
the control lines (assuming you call an active control line "one").

(I've just GOT to be more careful... I've only been doing this for 20 years!)

Rob Warnock

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