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Overview: Servers are central computers that can be located anywhere in the world.
To use or maintain them, they need to be accessed through so-called dumb
terminals or thin clients or PCs running a terminal emulation,
e.g. VT420 or xterm.
The VT420 terminals originate in past when DEC was a major manufacturer of mainframe computers running the VMS operating system. VT420 terminals were used to access these servers. Nowadays terminals are emulated (simulated) by software, hence software that takes the function of those VT420 terminals is now called a VT420 emulator. Such software (like ZOC Terminal) is available to access server operating systems like OpenVMS, Linux and others.
Background of VT420 Terminal Emulation/Emulators
In computing, a DEC VT420 is a reliable all purpose terminal type
for access to a variety of computer systems.
The VT420 (Video-Terminal 220) was originally developed by DEC, a former competitor of IBM and maker of computer equipment and mainframe computers. The VT420 was originally sold as a physical stand-alone terminal for the VAX mainframes running the VMS operating system.
In the past (before the PC), a terminal was to be a device consisting of a physical screen and keyboard. The terminal was connected to a central server and provided a user with the means to enter data into the server and see results that the server sent back to the user's screen. Thus the users had (rather inexpensive) terminals on their desk to provide them with shared access to a (prohibitively expensive) central computer.
However, with inexpensive PCs being available anywhere, the use of a single purpose screen and keyboard to access a server has become an outdated concept. Instead, these days a program on a PC can, emulate (simulate) the functions of a VT420 terminal. A software that does this, is called a terminal emulator.
Such software is still in demand though, because the server software once running on mainframes is now running on affordable machines (e.g. OpenVMS or Linux). The mark of a good emulation, besides adding features to the original terminals, is to inpret the data stream in exactly the same way as the original terminal did (there is even a software dedicated to testing this for VT terminal emulators).
VT200 Technical Details
As all terminal types, VT420 is a standard that allows the server to send text to the user's screen.
By embedding special controls in the text, rather than just displaying text from left to right and top
to bottom, these codes also allow control over the placement and display charachacteristics
(location, color, etc.) of the text.
E.g. in order to send the text "this is an error!" to the user's screen with the word "error" highlighted, the host would send This is an ^[1merror^[0m! to the VT420 terminal. Rather than displaying all the text, the VT420 terminal will interpret ^[1m and ^[0m as commands that tell it to highlight the text that is received between them.
Here is a list of basic VT420 control codes:
ESC [ n @ → Insert n (Blank) Character(s)
ESC [ n A → Cursor Up n Times
ESC [ n B → Cursor Down n Times
ESC [ n C → Cursor Forward n Times
ESC [ n D → Cursor Backward n Times
ESC [ n ; n H → Cursor Position [row;column]
ESC [ n I → Cursor Forward Tabulation n tab ston (default = 1)
ESC [ n J → Erase in Display ( n= 0/1/2 → below/above/all)
ESC [ n K → Erase in Line ( n= 0/1/2 → left/right/all)
ESC [ n K → Set text attributes ( n= 1, highlight; n= 30-37 foreground color)
(ESC equals character hex 1B or ^[ )
The VT420 emulation extended the possibilities of earlier terminal definitions like DEC VT100 or VT100 to allow the server to control special characteristics that go beyond the requirements of initial vt100 terminals, e.g. to support additional keys (like F5 - F12 function keys) that were not available on the VT100/VT102 series terminals.
Another improvement in the VT420 terminals was the availability of country specific character sets and thus the ability to render text in European languages correctly (e.g. the German word "über").
ZOC Terminal, a Modern VT420 Emulation for Windows and macOS
Today most 'terminals' are simply a software running on a PC. But many standard
can be used to emulate VT420 terminals in a basic way, but lack support for
certain subtleties in the way true VT420 terminals handled the incoming data.
Essentially, the basics of VT420 are easy to implement, which is why it is supported
in most terminal software to some extent, but the subtleties are hard to get right.
ZOC however has a long history of VT420 emulation and correctly implements VT420 details, that are rather arcane. Thus it lets you access servers via a telnet or SSL/SSH connection using a top notch VT420 emulation.The ZOC telnet/SSH client also includes a number of other useful features. It comes with a modern multi-tabbed user interface and is highly configurable. Beyond that it includes the usual terminal features such as keyboard redefinition and scroll back buffer. It also has some very advanced and unique features such as a powerful script language and automatic triggering of actions based on received or typed text. This terminal emulation software also supports vt102, vt220, vt420 and several types of ansi as well as Wyse, TVI, and Sun's CDE. ZOC also features major file transfer protocols like X-, Y- and Zmodem as well as Kermit and others. All these are offered in solid implementations that leave nothing to be desired.
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ZOC Terminal V8.06.5