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December 12, 2017 7:32 pm

Guide to VT220 Emulation

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VT220 Emulation uses the telnet protocol to create a TCP/IP connection to a remote host typically running the Unix, AIX, IBM i or Linux operating system. Originally, video terminals were used to communicate with these systems. The most established manufacturer of VT terminals was Digital Equipment Corporation (DEC). The first of these types of terminals was the VT05, released in 1970.

As time progressed and VT emulation gained in popularity, other manufacturers began producing video terminals to compete with the DEC VT05. The result was a splintered market with a multitude of terminals each using a proprietary set of escape sequences to communicate with the host system.

User frustration with functional inconsistencies and system incompatibility resulted in the development of the ANSI device control standard. The first DEC terminal to support the ANSI standard was the VT100. The DEC VT220 terminal was introduced to the market in 1983, and included an extended keyboard and the ability to redefine display characters. It was followed by releases of the widely used VT320 and VT420 terminals that included graphics support.

By the late 1980’s, PC use had gained a considerable foothold within business organizations. Users now needed a way to communicate with mainframes from their desktops without the need for a separate display terminal. Terminal emulation software was developed to fill this need. Using VT220 emulation, users who had previously operated a DEC VT220 terminal for accessing their host system remotely, could access it from their desktop PC instead.

VT220 emulation supports both 7-bit and 8-bit coded character sets. The 7-bit character set consists of the first 128 ASCII characters only. The 8-bit character extends the 7-bit set by accommodating additional multinational and graphical characters.

One feature that is available with VT220 emulation is the answer back message. The answer back message initiates a question and answer progression between the PC and the host system. This feature can be used to pass the identity of the PC to the host system without user interaction. Hexadecimal values can be used to include function keys in the answer back message. For example, to send the enter key (^M), use the hexadecimal equivalent of ^M preceded by the ~ character, which would be ~0D.

Another feature that can be used with VT220 emulation is local echo. With local echo enabled through the terminal emulator, each keyboard command that is entered is echoed back on the display screen. For example, if a user typed p3, pp33 would be displayed on the screen. The first instance of “p” would be the “p” that the user typed, and the second instance is the “p” returned by the host. Local echo can be used for diagnostic purposes.

Most terminal emulators will also allow users to configure a scroll back buffer. With the scroll back buffer configured, the user can scroll back up the screen the specified number of lines to previously issued commands. Some terminal emulators also allow users to configure the speed with which the buffered data will scroll across the screen.

VT220 emulation is often used to access IBM pSeries or System P servers. This line of servers was originally named RS/6000, eventually became the IBM pSeries line, and has now been replaced by IBM PowerT Systems. Using VT220 emulation in conjunction with these servers from IBM offers virtualization, consolidation and energy management technologies that can significantly reduce the costs associated with enterprise data and application management.

Zephyr Development Corporation is a highly respected developer of advanced terminal emulation and host integration solutions for Microsoft Windows. More about Zephyr’s PASSPORT VT220 emulation: http://www.zephyrcorp.com/vt220-emulation.htm
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