You should be aware of the considerations when using VTxxx emulation.
As with any emulation type, you should be aware of certain considerations before using the VTxxx full-screen mode with your Telnet Server. These considerations include security concerns as well as possible error conditions and indicator lights. Become familiar with these considerations to better understand how to use VTxxx full-screen mode.
In addition to security concerns, there are many other concerns to consider before using VTxxx full-screen mode with your Telnet server. When using VTxxx full-screen mode, you should be aware of the following concerns:
The number of sign-on attempts allowed increases if Telnet automatically configures virtual devices. The number of sign-on attempts is equal to the number of system sign-on attempts allowed multiplied by the number of virtual devices possible.
The QMAXSIGN system value defines the number of system sign-on attempts allowed. The QAUTOVRT system value defines the number of virtual devices Telnet can create.
The iSeries™ server supports 5250 pass-through. 5250 pass-through is similar to Telnet, but runs on an Systems Network Architecture (SNA) protocol network rather than a TCP/IP network. 5250 pass-through uses virtual displays to direct output to the physical devices just as Telnet does. In 5250 pass-through, the iSeries server automatically creates virtual devices in the same way as it does for Telnet. Therefore, the Devices System Value controls the number of automatically configured virtual devices for both 5250 pass-through and Telnet.
The system request processing for the VTxxx sessions is slightly different than that for a normal 5250 workstation.
When the System Request key is pressed on a 5250 workstation, a system request command line appears at the bottom of the display. If you press the Enter key, the System Request menu appears.
For VTxxx sessions when you call the system request function, the System Request menu displays immediately.
Certain error conditions cause a 5250 keyboard to lock and an error code to display on the message line. An example of such a condition is typing when the cursor is not in an input field. For VTxxx sessions, these errors cause a bell to sound on the VTxxx workstation and the keyboard to remain unlocked.
Certain iSeries applications also lock the 5250 keyboard and turn on the 5250 input-inhibited light. The user must press the Error Reset key before the keyboard unlocks. For VTxxx sessions, the locking of the 5250 keyboard causes a bell to sound on the VTxxx terminal whenever a key is pressed. To unlock the keyboard, the VTxxx key that is mapped to the Error Reset key must be pressed. In the default VTxxx keyboard map, the CTL-R key maps to the Error Reset key.
When the system negotiates VTxxx support, the Telnet server transmits screens that are a maximum of 24 rows by 80 columns. The VTxxx client system sees these screens in much the same way as they appear on a 5251 Model 11 workstation. However, there are some differences.
A 5251 workstation has indicator lights on the right side that indicate: System Available, Message Waiting, Keyboard Shift, Insert Mode, and Input-Inhibited.
The VTxxx server support emulates the System Available light by putting an asterisk in column 80 of row 9. For Message Waiting, Insert Mode, and Input-Inhibited lights, the asterisk appears in column 80 of rows 11, 13, or 15. When an asterisk appears, the asterisk overwrites the character that was previously displayed at that screen location. By default, the VTxxx server does not display the indicator lights. You can enable or disable these indicators by typing the key sequence that is mapped to the toggle indicator lights function. The default key sequence for this function is ESC-T.
As an iSeries Telnet user, you should be aware of physical and operational differences between VTxxx and 5250 terminals.
The 5250 is a block mode terminal. Data typed on a 5250 is accumulated in a buffer and only sent to the iSeries server when an AID (attention identifier) key ispressed. An AID key on a 5250 keyboard is a key that initiates a function. The following list shows the AID keys on a 5250 keyboard:
VTxxx terminals operate in a character mode. Characters transmit immediately to the host when a key is pressed.
Another difference is the way the data arrives on the display. The system writes data to a VTxxx terminal one character at a time, and you see the data arrive as streams of characters. With the 5250, the system writes data in blocks, and all or part of the display changes at once.
You should avoid using the 5250 cursor movement keys. Instead, you should use the function keys associated with the *CSRUP, *CSRDOWN, *CSRRIGHT, and *CSRLEFT keywords. By default these are keys F13, F14, F15, and F16. If you use the 5250 cursor movement keys, the VTxxx application you are using might not function as expected. This is because the results of using these keys do not transmit to the remote system until an attention identifier (AID) key is pressed.
For example, using Telnet to the RS/6000® and obtaining VT220 emulation, the SMIT command provides a menu driven interface to AIX®. Here the function keys associated with *CSRxx keywords perform as you will expect the cursor movement keys to do. However, the 5250 cursor movement keys, while physically moving the cursor down the screen and correctly selecting the SMIT option, do not cause the selected option to be highlighted. The highlighting in reverse image remains with the first option on the SMIT menu, regardless of the key position.
Typing a control character on an iSeries keyboard is different than typing a control character on an actual VTxxx terminal. On a VTxxx terminal, press and hold down the control key while pressing the character associated with the control function.
When using the iSeries Telnet support, the equivalent is achieved by typing a 2 character control indicator followed by pressing the function key associated with the Send without Carriage Return (*SENDWOCR) default function (the F11 key). For example, if the default keyboard map and the default STRTCPTELN command parameters are in effect, the VTxxx Control-C function can be entered by typing &C followed by pressing the F11 key. <F12> can also enter this function, using the default keyboard map. In case you are using an application where <F12> is remapped, this example is included, and illustrates the principle of the *SENDWOCR key.
Use the CTLCHAR parameter of the STRTCPTELN command to select the character used to indicate a control character. The default is &. The &C characters must be the last characters typed before pressing the *SENDWOCR function key or the &C is not interpreted as a control character. A control character only transmits when the *SENDWOCR function key is pressed. You can assign frequently used VTxxx control characters to a function key. The following is a descriptive example of the Ctrl-C command. When using a Telnet client to connect to an RS/6000 system, the system typically negotiates VT220 emulation. The Ctrl-C sequence is an important one in AIX to end long running commands, such as PING. It is, therefore, important that you know how to do this before issuing any RS/6000 commands. By default the sequence is &C<F11>. Note that you need to enter these keys quickly, and it might take several attempts before the RS/6000 task accepts the input.
Press function key that is associated with the *HIDE function, (F6 on the default keyboard map) if you do not want to display the typed characters. Use this function when typing a password.
If you want the characters that have been typed to be sent to the remote system for processing without pressing the Enter key, you should press the function key associated with the *SENDWOCR function (F11 on the default keyboard map).
It is often useful to be able to recall previously entered commands. On the iSeries server, F9 often provides this function. On AIX, this can be activated by typing the command set -o vi and pressing Enter. After this, you can start retrieving commands with the sequence Esc-K. To perform this sequence using the default keyboard map while in VTxxx emulation, you should usethesequence <F5>k<F11>. The Esc character starts the command retrieval. Then use the k to retrieve further commands. While operating in this mode, the commands H for right, L for left, X for delete, I for insert, and R for replace apply. The sequence<F5>i<F11> switches this facility off.
The character in the position just before the cursor position will always be blank. The actual character saves internally and shows when the display refreshes with the cursor in a different position.
A VTxxx application that uses row 1, column 1 of the display does not work the same when using iSeries Telnet client support. Most 5250-type display stations do not allow input to row 1, column 1. If the VTxxx application positions the cursor at row 1, column 1, the iSeries server puts the cursor at row 1, column 2 automatically.
Due to architectural differences, the system ignores certain unsupported commands or sequences. An example is downstream loadable character sets.
Telnet VTxxx full-screen mode supports the following screen sizes:
A VTxxx terminal supports the following attributes:
The 5250 data stream supports the previous attributes so that a 5250 display station can represent all of the VTxxx attributes. However, there are some limitations:
A VT100 terminal has a L1 indicator that can be programmed for different applications. This indicator is not emulated by the iSeries Telnet support.