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Appendix I, Terminals and Terminal Emulators


VT Terminals

Serial communications

Serial Modems


PC Terminals

X Terminals





Terminals and Terminal Emulators, History and Background

Before there were high speed network connections at everyone's desk and high speed modems to make mobile or home network connections feasible, large scale use of computers was still happening, but it was based on a slower and more economical communication and user interface. Instead of presenting pictures, graphical user interfaces and serving files for remote use, computers kept their files local, on their own disks, and communicated with a simple serial protocol, RS232, using ASCII code text characters as a user interface. The text communications, although slow and simple by today's standards, were not ineffective. In fact, they were highly efficient and economical. Using serial communications, precise ways of doing all computing tasks (except for graphics of course) were developed which are still preferred by many programmers today. The same computer languages based on ASCII text are still used to write most computer programs today.

To the user, the center of the serial based computer universe was the character cell terminal. Even though the computer might have taken up an entire building, so effective was the serial, character cell terminal interface, that the user imagined that the computer was the little terminal display box and keyboard on their desk. And since almost everything that was done with a computer was done with a serial, character cell terminal, the image of the terminal as the computer was true in a real sense. So for decades, starting with teletypes and VT terminals with CRT displays and finally to graphical X-window terminals and PC emulator software, the character cell interface of the terminal has been used intensively.

The evolution and development of terminals and terminal emulator software was not orchestrated by a single organization or company so it is not without some odd parts and paradoxes. But, there are some standards for terminals that have surfaced over the years. Among the most important are the serial line communication standard, RS232, the Internet TELNET standard and the VT terminal standard.

The RS232 standard defines how terminals communicate serially with a computer. Data throughput many times higher than reading or typing speed are easily attained using as few as 3 conductors or copper wires. The success of serial communications was due in part to the fact that under this standard it was for the most part no more difficult to install serial cables than it was to install telephone lines. The serial standard also worked well for telephone modems which made remote computer access possible.

When network connections became more common the Internet TELNET standard was developed so the serial information could be transmitted using TCPIP network packets instead of serial cables. The economy and efficiency of the character cell terminal interface made it useful if not absolutely necessary in over-used, stressed out TCPIP network environments. Even if the computing task is the same, a terminal interface with a computer is typically passing only 0.001 megabytes of data on the network while file and application serving interfaces are passing multiple megabytes of data. File and application serving works fine if the network bandwidth or through put is good. But, if the network is overloaded, the terminal keeps on working while the network applications pause.

The most important of all standards a character cell terminal interface was the VT terminal standard. The VT terminal standard characterizes how a terminal operates and is an unofficial bench mark for the function of terminals and terminal emulators. Any computer which communicates using serial RS232 protocol is designed to function with a VT type terminal. Any computer which supports TELNET is designed to function with a VT type terminal.

Digital Equipment Corporation, DEC, entered the character cell terminal market with successive terminal models designated by the letters VT followed by a number, (05, 50, 100, 220, 320, 420, 520, etc.) The terminals were cheap, reliable, easy to manage, and extremely popular. As a result, VT terminals became a standard. Vendors of competing terminals and terminal emulation software would designate their product as performing as a VT420, VT525, etc. Some lived up to this claim, some didn't, some still don't and some never had a chance due to hardware limitations.



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

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