Terminal (telecommunication)

Terminal (telecommunication)

In the context of telecommunications, a terminal is a device which is capable of communicating over a line. Examples of terminals are telephones, fax machines, and network devices - printers and workstations.

The mobile data terminal (MDT) is a device used in the field of telematics.


Emulation of terminals by computer programs

Many computers use a serial access program to communicate with other computers via telephone line or serial device, thus emulating a computer terminal. HyperTerminal is a widely distributed terminal emulator.

When the first Macintosh was released, a program called MacTerminal was used to communicate with many computers, including the IBM PC.

Dec Terminal was one of the first Terminal programs for the popular Altair.


Terminals can operate in various modes, relating to when they send input typed by the user on the keyboard to the receiving system (whatever that may be):

character mode (a.k.a. character-at-a-time mode)
In this mode, typed input is sent immediately to the receiving system.[1]
line mode (a.k.a. line-at-a-time mode)
In this mode, the terminal provides a local line editing function, and sends an entire input line, after it has been locally edited, when the user presses a return key.[1] A so-called line mode terminal operates solely in this mode.[2]
block mode (a.k.a. screen-at-a-time mode)
In this mode, the terminal provides a local full-screen data function. The user can enter input into multiple fields in a form on the screen (defined to the terminal by the receiving system), moving the cursor around the screen using keys such as Tab and the arrow keys and performing editing functions locally using insert, delete, ← Backspace and so forth. The terminal only sends the completed form, comprising all of the data on the screen, to the receiving system when the user presses an Enter key.[3][4][1]

Note the distinction between the return and the Enter keys. In some multiple-mode terminals, that can switch between modes, pressing the Enter key when not in block mode does not do the same thing as pressing the return key. Whilst the return key will cause an input line to be sent to the host in line-at-a-time mode, the Enter key will rather cause the terminal to transmit the contents of the character row where the cursor is currently positioned to the host, host-issued prompts and all.[3]

Different computer operating systems require different degrees of mode support when terminals are used as computer terminals. The POSIX terminal interface, as provided by Unix and POSIX-compliant operating systems, does not accommodate block-mode terminals at all, and only rarely requires the terminal itself to be in line-at-a-time mode, since the operating system is required to provide canonical input mode, where the terminal device driver in the operating system emulates local echo in the terminal, and performs line editing functions at the host end. Most usually, and especially so that the host system can support non-canonical input mode, terminals for POSIX-compliant systems are always in character-at-a-time mode. In contrast, IBM 3270 terminals connected to MVS systems are always required to be in block mode.[5][6][7][8]


What supports what

  1. ^ a b c Bolthouse 1996, p. 18.
  2. ^ Bangia 2010, p. 324.
  3. ^ a b Diercks 2002, p. 2.
  4. ^ Gofton 1991, p. 73.
  5. ^ Raymond 2004, p. 72.
  6. ^ Burgess 1988, p. 127.
  7. ^ Topham 1990, p. 77.
  8. ^ Rodgers 1990, p. 88–90.

Sources used

  • Bangia, Ramesh (2010). "line mode terminal". Dictionary of Information Technology. Laxmi Publications, Ltd. ISBN 9789380298153. 
  • Bolthouse, David (1996). Exploring IBM client/server computing. Business Perspective Series. Maximum Press. ISBN 9781885068040. 
  • Burgess, Ross (1988). UNIX systems for microcomputers. Professional and industrial computing series. BSP Professional Books. ISBN 9780632020362. 
  • Diercks, Jon (2002). MPE/iX system administration handbook. Hewlett-Packard professional books. Prentice Hall PTR. ISBN 9780130305404. 
  • Gofton, Peter W. (1991). Mastering UNIX serial communications. Sybex. ISBN 9780895887085. 
  • Raymond, Eric S. (2004). The art of Unix programming. Addison-Wesley professional computing series. Addison-Wesley. ISBN 9780131429017. 
  • Rodgers, Ulka (1990). UNIX database management systems. Yourdon Press computing series. Yourdon Press. ISBN 9780139455933. 
  • Topham, Douglas W. (1990). A system V guide to UNIX and XENIX. Springer-Verlag. ISBN 9780387970219. 

External links