Text-Terminal-HOWTO: Types of Terminals
2. Types of Terminals
2.1 Dumb TerminalsThere are various conflicting definitions of "dumb terminal" but
as time goes by, more and more terminals are called dumb. This
document mainly covers text terminals which display only text on the
screen. It might be titled "Dumb-Terminal-HOWTO" but in some
magazines articles any terminal, no matter how smart, including ones
which present a full graphical user interface (GUI), are called dumb.
If all terminals are "dumb" then there is no point of prefixing the
word "dumb" to terminal (except as a sales pitch to sell computers or
the like in place of "smart" terminals). Due to the ambiguous meaning
of "dumb terminal" it is not classified here as a type of terminal.2.2 Text TerminalsFor a text terminal, a 2-way flow of information between the
computer and the terminal takes place over the cable that connects
them together. This flow is in ASCII bytes where each byte usually
represents a character. Bytes typed at the keyboard go to the
computer and most bytes from the computer are displayed on the
terminal screen. Special bytes (or sequences of bytes) from the
computer tell the terminal where to move the cursor to, what to erase,
where to begin and end underlining and/or blinking and/or bold, etc.
There are often hundreds of such special commands and many terminals
can even change fonts.The communication uses characters (letters) encoded using a code chart
for the character set being used. Usually, the first 128 bytes out of
256 possible bytes use ASCII codes. Terminals for Unix-like systems,
normally connect to computers via a cable running between the
asynchronous serial ports (RS-232-C = EIA-232-D) of the host computer
and terminal. Sometimes the connection is via modem or terminal
server, etc.Other names for text terminals are "serial terminal", "character-cell
terminal", "ASCII/ANSI terminal", "asynchronous terminal", "data
terminal", "video terminal" and "video display terminal" (VDT). In
olden days "video display unit" (VDU) was used for terminals but
strictly speaking, it excludes the keyboard."Block mode" was used exclusively by old IBM mainframe terminals but
many modern terminals also have this capability (which is not used
much). The characters you type are temporarily retained in the
terminal memory (and may possibly be edited by a built-in editor at
the terminal). Then when the send key (or the like) is pressed, a
block of characters (sometimes just a line of characters) is sent to
the computer all at once. Block mode (as of late 1998) is not
supported by Linux. Block mode makes the terminal a block device
(and not a character device). See section Block Mode.2.3 Graphics TerminalsTo a limited degree some ASCII symbols can provide graphics on
text terminals. One may form arrows <--- and draw boxes with _ and
|. With special graphic character sets, even more is possible. None
of these are really graphics terminals. However, the term "graphics
terminal" is sometimes applied to all text-only terminals since text
is a limited form of graphics.There are two basic types of graphics displays: raster and vector
(rarely used). Raster graphics (bit-mapped) puts dots on the screen
by horizontal scan lines drawn by an electron beam (or by activating
pixels or dots on a flat screen). Vector graphic displays use smart
electronics to draw lines and curves with an electron beam that can
move in any direction. Vector graphics draws high quality lines
without zig-zags but is both rare and expensive. Raster graphics is
almost universally used today. For PC's, images encoded in vector
graphic format are sometimes used but they are translated to raster
graphics format for display (with a drop in image quality).Serial Line Graphics TerminalsMost of this document also applies to these. Most of these can
also function as text terminals. The protocols for such graphics
include: Tektronix Vector Graphics, ReGIS (DEC), Sixel (DEC), and
NAPLPS (North American Presentation Level Protocol Syntax).Fast Graphics Terminals (often known by other names)None of these covered in this document. A terminal that deserves
to be called smart is a graphics terminal which can rapidly display
full-screen graphics just like a PC monitor. It will also have a
mouse. Bytes sent to it often represent bit-maps for pictures (and
other graphics). It will often use a high-speed connection to the
computer using twisted pair or coax cable. X-Window terminals are
such devices. See the link to Xterminal-HOWTO at Related HOWTO'sFor displaying a MS-Windows GUI there are at various types of
interfaces and terminals: Winterm using WinFrame software from Citrix
is one. Another (based in part on Citrix's code) is Hydra (code name)
by Microsoft, also known as "Windows Terminal Server" which works with
versions 4 or higher of MS Windows NT. Citrix uses its ICA protocol
and has created an add-on to Hydra known as pICAsso so that WinFrame
(ICA) based terminals can use the Hydra system. Hydra is also
multi-user. There is also the "MultiConsole Personal Terminal" by
Unbounded Technologies and Tektronix had its own multi-user interface
but will now support Hydra. A magazine article in 1997 called Winterm
a "dumb terminal" but it's really pretty smart. Such terminals are
often called "thin clients", but some thin clients are more that just
terminals as they can execute Java code sent to them, etc.2.4 Quasi-Terminals (= Quasi-Computers)"Quasi" is my terminology (not standard). These are neither true
computers nor terminals but are something in-between. Network
Computers (NC's) are computers with a CPU but no hard Disk. They are
full-graphics and connect to a server computer. They are different
from terminals since the programs they run execute on their own CPU
chips. Java code may be sent to them for execution. They should work
on IP networks and might work under a server running Linux. Wintel
established a "NetPC" which, unlike the NC, is almost a PC computer.
However, it has no removable disks so users can't install their own
software or obtain copies of anything.2.5 Emulation on a PCSince a PC has a screen and keyboard (as does a terminal) but also
has much more computing power, it's easy to use some of this computing
power to make the PC computer behave like a text terminal. This is
"terminal emulation". It's not strictly a type of terminal since
emulation could emulate any of the above types (but the most common is
the emulation of a text terminal). See Terminal Emulation
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