68 lines
3.3 KiB
HTML
68 lines
3.3 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
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<!--Converted with LaTeX2HTML 96.1-c (Feb 29, 1996) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds -->
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<HTML>
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<HEAD>
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<TITLE>Introduction to Serial Devices</TITLE>
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</HEAD>
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<BODY LANG="EN">
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<A HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR>
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<B> Next:</B> <A HREF="node56.html">Accessing Serial Devices</A>
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<B>Up:</B> <A HREF="node53.html">Setting up the Serial </A>
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<B> Previous:</B> <A HREF="node54.html">Communication Software for Modem </A>
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<BR> <P>
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<H1><A NAME="SECTION006200000">Introduction to Serial Devices</A></H1>
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<A NAME="serialttys"></A>
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The devices a kernel provides for accessing serial devices are
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typically called <em>ttys</em>. This is an abbreviation for <em>Teletype</em>,
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which used to be one of the major manufacturers of terminals in the
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early days of Unix. The term is used nowadays for any character-based data
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terminal. Throughout this chapter, we will use the term exclusively to
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refer to kernel devices.
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<P>
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distinguishes three classes of ttys: (virtual) consoles,
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pseudo-terminals (similar to a two-way pipe, used by application such as
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X11), and serial devices. The latter are also counted as ttys, because
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they permit interactive sessions over a serial connection; be it from a
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hard-wired terminal or a remote computer over a telephone line.
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<P>
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Ttys have a number of configurable parameters which can be set using the
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ioctl(2) system call. Many of them apply only to serial devices,
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since they need a great deal more flexibility to handle varying types of
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connections.
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<P>
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<A NAME="2306"></A>
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<A NAME="2307"></A>
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Among the most prominent line parameters are the line speed and parity.
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But there are also flags for the conversion between upper and lower case
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characters, of carriage return into line feed, etc. The tty driver may
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also support various <em>line disciplines</em> which make the device
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driver behave completely different. For example, the SLIP driver for
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is implemented by means of a special line discipline.
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<P>
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<A NAME="2310"></A>
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<A NAME="2311"></A>
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<A NAME="2312"></A>
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<A NAME="2313"></A>
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There is a bit of ambiguity about how to measure a line's speed. The
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correct the term is <em>Bit rate</em>, which is related to the line's transfer
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speed measured in bits per second (or bps for short). Sometimes, you hear
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people refer to it as the <em>Baud rate</em>, which is not quite correct.
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These two terms are, however, not interchangeable. The Baud rate
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refers to a physical characteristic of some serial device, namely the
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clock rate at which pulses are transmitted. The bit rate rather
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denotes a current state of an existing serial connection between two
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points, namely the average number of bits transferred per second. It
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is important to know that these two values are usually different, as
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most devices encode more than one bit per electrical pulse.
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<P>
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<HR><A HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR>
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<B> Next:</B> <A HREF="node56.html">Accessing Serial Devices</A>
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<B>Up:</B> <A HREF="node53.html">Setting up the Serial </A>
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<B> Previous:</B> <A HREF="node54.html">Communication Software for Modem </A>
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<P><ADDRESS>
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<I>Andrew Anderson <BR>
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Thu Mar 7 23:22:06 EST 1996</I>
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</ADDRESS>
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</BODY>
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</HTML>
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