VTxxx server support enables Telnet client users to log on and run 5250 full-screen applications, even though VTxxx full-screen support is negotiated.
The Telnet client application must be able to negotiate VTxxx terminal support. When VTxxx full-screen mode is negotiated, the Telnet server is responsible for mapping 5250 functions to VTxxx keys and vice versa.
Although the Telnet server supports VTxxx clients, this is not the preferred mode to use because the VTxxx terminal is a character-mode device. The i5/OS® operating system is a block-mode system. Most Telnet implementations support a TN3270 or TN5250 client that should be used when connecting to a Telnet server.
In general, when a key on a VTxxx terminal is pressed, the hexadecimal code associated with that key immediately transmits to the Telnet server. The Telnet server must process that keystroke and then echo that character back to the VTxxx terminal where it is displayed. This results in a large amount of overhead associated with each keystroke. In contrast, the 5250 and 3270 block mode devices buffer all keystrokes at the client system until an attention identifier (AID) key is pressed. When an AID key is pressed, the client sends the buffered input to the server for processing. The block-mode devices result in less overhead per keystroke and generally provide better performance than a character-mode device, such as the VTxxx terminal.
VTxxx delivers the data between the two systems as ASCII.
After you have completed the general configuration of the Telnet server, you need to complete a few additional steps to enable server support for VTxxx full-screen mode.
Full-screen mode is a block mode as opposed to a line mode. Line mode is when data transmits one line at a time, while block or full-screen mode transmits the whole screen at one time.
Complete the following tasks to configure the server for VTxxx full-screen mode:
When the Telnet server automatically creates virtual display devices, it uses the QKBDTYPE system value to determine the keyboard type for the virtual device.
If the initial creation of the virtual device fails using the QKBDTYPE system value, the Telnet server tries to create the device again, using a keyboard type value of USB. If the second attempt to create the keyboard type fails, then the system sends a message (CPF87D7) to the QTCPIP job log. This message indicates that the system cannot automatically create the virtual device. The system also sends the message to the system operator message queue.
When a Telnet session negotiates in VTxxx full-screen mode, the system uses a default keyboard map. To display the default keyboard map for VTxxx, use the Display VT Keyboard Map (DSPVTMAP) command. To change the VTxxx keyboard map, use the Change VT Keyboard Map (CHGVTMAP) command or the Set VT Keyboard Map (SETVTMAP) command.
The numeric keypad table shows the keys on the auxiliary keypad that normally transmit the codes for the numerals, decimal point, minus sign and comma.
The editing keypad table shows the keys that transmit codes for the editing keypad keys.
Because the VTxxx keyboard does not have the same keys as a 5250 keyboard, a keyboard mapping must exist between the VTxxx keys and the i5/OS functions. The system assigns a default keyboard mapping when a VTxxx session is first established. In some cases, there can be more than one key or key sequence that maps to a particular i5/OS function. In these cases, you can use any of the defined keys to call the required i5/OS function.
The default network virtual terminal type parameter specifies the mode to use when the Telnet server is not able to negotiate one of the supported terminal types.
To set the value of the default network virtual terminal to either *VT100 for VT100/VT220 mode, or *NVT for ASCII line mode, complete the following steps:
The Telnet server uses default ASCII-to-EBCDIC and EBCDIC-to-ASCII mapping tables based on the coded character set identifier (CCSID) parameter in the TCP/IP Telnet attributes. The default is to use the DEC multinational character set (*MULTINAT). Other 7-bit and 8-bit ASCII CCSIDs, and any of the 7-bit DEC national replacement character sets are also acceptable to use.
For VT220 8-bit mode, the mapping tables are not available. In this mode, the system uses the DEC replacement character sets. For the VT220 7-bit mode, you can use either the mapping tables or the DEC replacement character sets.
There are three ways to change the default. You can change the CCSID parameter, specify different values for the VTxxx outgoing (TBLVTOUT) and incoming tables(TBLVTIN), or change the default tables for the current session.
Another way to access this command is to use option 2 on the CHGTCPTELN command.
Related concepts
VTxxx Telnet client sessions