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Appendix I, Terminals and Terminal Emulators


VT Terminals

Serial communications

Serial Modems


PC Terminals

X Terminals







VT Terminals

Early in the 1970's Digital Equipment Corporation (DEC) started manufacturing and selling video terminals with the designation, VT. The earliest VT's were big, clumsy, hot, expensive and unreliable, to say the least. But, in spite of their shortcomings, they were liberating to programmers who for years had suffered writing programs with paper tape, computer cards, and teletypes. (A teletype is sort of an electric typewriter with a serial interface to a computer.) The VT terminal screen allowed a programmer to see an entire page of a program, move a cursor to a spot on that page, make a change, and to immediately see the entire corrected page. It allowed programmers to see changes in programs that they could only have visualized in their minds before the video terminal. These new graphics features were possible because of the video display and internal terminal screen controls which allow the video display to be controlled by the editor program.

To take advantage of the new VT technology, the old, line editor programs were enhanced to be "page" editor programs which were written to use the VT video features. Later, editors written specifically for video display terminals were introduced. The VT terminal was also continuously improved by adding internal screen or graphics controls for the new page editors. These controls were improved in each new model of VT terminal making each model more functional with the editor and useful to the user.

VT terminal video controls, which were invisible to the user, were constructed with ASCII text ESCAPE CHARACTERS which consisted of the ESCAPE character followed by some ASCII text characters. The ESCAPE CHARACTERS notified the terminal that the following characters were video control commands and not to be displayed to the user as text. Each ESCAPE CHARACTER had a meaning to the terminal model for which was intended but not necessarily to any other terminal. So as text editor programs and terminal video controls improved, the older terminals became incompatible with the newer, more powerful page editor programs.

The first VT terminal model which contained all the graphics features required to take advantage of today's applications and editors was the VT102 terminal, an enhanced version of the VT100 terminal. The VT100 standard is still an important terminal standard today as a sort of lowest common denominator of terminal function. Older versions of operating systems (like unix, OpenVMS) will almost always recognize VT100 and function well with it, especially older unix systems. Soon after its introduction a new series of terminals, the VT200 series was introduced. This is the more common standard now, due to enhancements and extensions from the VT100 standard. If you're not sure what what type of terminal your operating system or application supports, starting with VT200 or VT220 setting is a good bet.

Following the VT220 were the VT320, VT420, VT520, and VT525 models, each with new controls and features. Use of these newer VT models doesn't mean that the user has to worry about the compatibility of video controls as each newer VT terminal model is downward compatible with older terminal applications. If your application or OS thinks it's talking to a VT220 and you're really using a VT525, everything should still work fine. If you are using OpenVMS, it can be configured to identify which type of terminal is being used to log on to the system and then use the appropriate software for that model of VT terminal (e.g. DCL command: SET TERMINAL/INQUIRE ).

The following is a description of each VT model and it's advantages.

VT220, broke the size, reliability, and power consumption barriers left by the older terminal models. Also had a much improved command set. Introduced a new, trim keyboard, the LK-201, which influenced the design of all future keyboards.

VT320, improved reliability, lower power, and price. Was the first terminal to use DEC's Modified Modular Jack, MMJ, for serial connections. MMJ was like a RJ-11 telephone jack but with an offset tab to keep people from plugging the terminal into the phone socket. It allowed anyone to be able to connect and disconnect their terminal serial cables. Previous to the MMJ, terminals used the 25 pin, DB25 connector which was much too complicated, expensive, and unreliable.

VT420, more graphics, lower power consumption, and reliability enhancements. It had three productivity enhancements:

  1. ability to run two separate sessions either split screen or one on top of the other
  2. up to 144 line history in each session
  3. ability to cut and paste text within a session or from one session to another

The VT420 also pioneered ergonomic health features such as an adjustable raised screen angle and lower radiation emissions. It also introduced a new, more comfortable keyboard, the LK-401, which is still a very good keyboard today.

VT420 Terminal

In addition to these advantages, it should be pointed out that all of these VT terminals have these advantages over personal computers in the office place:

  1. The VT terminal's life cycle cost is a fraction of that of a personal computer, in purchase price, power, space, technical support, maintenance, and ease of use.
  2. Time and effort to resolve and fix problems is minimal because the VT terminal and keyboard have no software and very few user adjustable settings. This simplicity means that users may be easily supported by telephone. Hardware replacement is only slightly harder than replacing a light bulb further speeding repair and lowering costs.
  3. VT terminals have longer mean time between failures. They use less power than personal computers and do not have internal fans. (Fans are one of the most unreliable parts on electronics and they also make a lot of noise.)

© 2000 - 2008 -- CCSS, William A. Pedersen

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