117 lines
6.2 KiB
HTML
117 lines
6.2 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
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<TITLE> Text-Terminal-HOWTO: Types of Terminals</TITLE>
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<A HREF="Text-Terminal-HOWTO-3.html">Next</A>
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<H2><A NAME="s2">2.</A> <A HREF="Text-Terminal-HOWTO.html#toc2">Types of Terminals</A></H2>
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<H2><A NAME="ss2.1">2.1</A> <A HREF="Text-Terminal-HOWTO.html#toc2.1">Dumb Terminals</A>
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</H2>
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<P> There are various conflicting definitions of "dumb terminal" but
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as time goes by, more and more terminals are called dumb. This
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document mainly covers text terminals which display only text on the
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screen. It could have been titled "Dumb-Terminal-HOWTO". But in
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some magazines articles, any terminal, no matter how smart, including
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ones which present a full graphical user interface (GUI), are called
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dumb. If all terminals are "dumb" then there is no point of prefixing
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the word "dumb" to terminal (except as a sales pitch to sell computers
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or the like instead of terminals). Due to the ambiguous meaning of
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"dumb terminal" it is not classified here as a type of terminal.</P>
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<H2><A NAME="ss2.2">2.2</A> <A HREF="Text-Terminal-HOWTO.html#toc2.2">Text Terminals</A>
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</H2>
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<P> For a text terminal, a 2-way flow of information between the
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computer and the terminal takes place over the cable that connects
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them together. This flow is in bytes (such as ASCII) where each byte
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is an integer that usually represents a printable character. Bytes
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typed at the keyboard go to the host computer and most bytes from the
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computer are displayed on the terminal screen. Special control bytes
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(or sequences of bytes) from the computer tell the terminal where to
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move the cursor to, what to erase, where to begin and end underlining
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and/or blinking and/or bold, etc. There are often hundreds of such
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special coded commands and most real terminals can even change fonts.</P>
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<P>The communication uses characters (letters) encoded using a code chart
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for the character set being used. Usually, the first 128 bytes out of
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256 possible bytes use ASCII codes. Terminals for Unix-like systems,
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normally connect to computers via a cable running between the
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asynchronous serial ports (RS-232-C = EIA-232-D) of the host computer
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and the terminal. Prior to about 2004, most new PCs had serial ports,
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but today (2009) almost no new PCs come with serial ports. Sometimes
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the connection is via modem or terminal server, etc.</P>
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<P>Other names for "text terminal" are "general purpose terminal",
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"general display terminal", "serial monitor", "serial console" (if
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it's used like a console), "serial terminal", "dumb terminal",
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"character-cell terminal", "character terminal", "ASCII/ANSI
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terminal", "asynchronous terminal", "data terminal", "video terminal",
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"video display terminal" (VDT), and "green terminal" (since many used
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green displays). These names (especially "dumb terminal") are
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sometimes used to mean emulating a text terminal on a PC with a
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command line interface such as Linux. In olden days "video display
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unit" (VDU) meant text terminal but strictly speaking, it excludes the
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keyboard.</P>
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<P>"Block mode" was used exclusively by old IBM mainframe terminals but
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many modern terminals also have this capability (which is not used
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much). In block mode, the characters you type are temporarily
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retained in the terminal memory (and may possibly be edited by a
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built-in editor at the terminal). Then when one presses the send key
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(or the like) a block of characters (sometimes just a line of
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characters) is sent to the computer all at once. Block mode (as of
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late 1998) is not supported by Linux. See section
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<A HREF="Text-Terminal-HOWTO-23.html#block">Block Mode</A>.</P>
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<H2><A NAME="ss2.3">2.3</A> <A HREF="Text-Terminal-HOWTO.html#toc2.3">Graphic GUI Capabilities of Text Terminals </A>
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</H2>
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<P> While emulated text terminals don't display images, many real text
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terminals can display bit-mapped images, but not in color.
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Unfortunately, the popular image formats used on the Internet are not
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supported. Thus the display of images is seldom used. The protocols
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for terminal graphics include: Tektronix Vector Graphics, ReGIS (DEC),
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Sixel (DEC), and NAPLPS (North American Presentation Level Protocol
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Syntax).</P>
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<P>Even without bit-mapped images, ordinary text terminals can sort of
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display images. One may form arrows <--- and draw boxes with |__|,
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etc. With special graphic character sets that have a lot of special
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characters for line drawing, much more is possible. But even without
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a graphic character set, one may produce "ascii graphics" art. The
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term "graphics terminal" usually means a terminal that can display bit
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mapped images. However, this term is sometimes applied also to
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text-only terminals since text is a limited form of graphics.</P>
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<H3><A NAME="vector_graphics"></A> Graphics GUI displays </H3>
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<P> There are two basic types of graphics displays: raster and vector
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(rarely used). Raster graphics (bit-mapped) puts dots on the screen
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by horizontal scan lines drawn by an electron beam (or by activating
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pixels or dots on a flat screen). Vector graphic displays were
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intended to be used for monochrome screens that don't have any dots.
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They use smart electronics to draw lines and curves with an electron
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beam that can continuously move in any direction (just like a pen or
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pencil). True vector graphics draws high quality lines without
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noticeable zig-zags but is both rare and expensive. For more details
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see
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<A HREF="http://www.cca.org/vector/">http://www.cca.org/vector/</A>. Raster graphics is almost
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universally used today for both PCs and text terminals. For PCs,
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images encoded in vector graphic format can't be drawn as continuous
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lines due to the electronic limitations but they can be translated to
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raster graphics format for display (with a resulting drop in image
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quality).</P>
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