what is a VT host connection

royalcyber:

Terminal (both direct and emulated) access to the AS/400 line of systems comes in three general flavors -- 5250, 3270 and ASCII. 5250 is the fundamental "native" workstation type. 3270 is supported mostly because many mainframe sites also have numerous AS/400s as departmental or distributed processors.

But ASCII...?

AS/400s could have ASCII workstation controllers installed as an option. Sites that had ASCII terminals could bring in an AS/400 and continue to use their ASCII terminals. (I'm not sure if the workstation controllers could also be installed on iSeries after AS/400s were obsoleted, nor on any System i systems after iSeries.)

As hardware workstation controllers were being replaced by virtual controllers, the supported workstation types were also supported in the virtual controllers. For ASCII terminals, the supported terminal types were VT100 and higher.

Nowadays, even the current System i's still handle ASCII VT100 emulated terminals.

Although the terminal types are handled, it can bring complications because most display screens are written to interact with 5250 devices. This often means that special keys such as the various function keys aren't available at the terminal; e.g., <F23> might not exist on an ASCII terminal.

In order to control display files that are designed for 5250, a command is usually executed in the initial program of a session that attaches with an ASCII (or even 3270) device type. The command is SETKBMAP -- Set Keyboard Map. This allows you to set a keyboard translation that changes the meaning of keys that are pressed. Prompting the command and reviewing the help will give much of the background details.

(The command could also be executed in a routing program or even at a command line.)

In general, ASCII terminals are very "dumb" terminals. Their advantage has historically been that they are cheap. A 5250 terminal can be smart enough to validate certain kinds of user input errors at the terminal itself. For example, it's possible to restrict the characters that can be typed into a numeric field -- no alpha characters allowed.

Tom

royalcyber:

Ummm.... yes. But it would be unusual.

I can think of a number of ways to do it, and even a couple reasons to do it.

Workstation entries in interactive subsystems could restrict device types only to ASCII types. I suppose that routing programs could do likewise. Telnet initiation exit programs could query the telnet device type and reject the connection.

I suppose that one reason to do so would be to add a minor layer of security. If all valid connections are known to be via ASCII terminals, then there might be no desire to allow other terminal types to connect; this would stop any possible external connections that weren't aware of the restriction and attempted to connect via 5250 or 3270.

There are many emulators that work with VT100 (and higher). Simple telnet from a Windows command line should connect as VT100 for example.

Tom