A control character is a character that has special meaning for the terminal or host computer. The terminal can send and receive all standard control characters. When some control characters are transmitted, you cannot see their result on the display. Others, however, cause visible changes in the display. For example, the carriage return control character causes the cursor to return to the left margin; the line feed control character causes the cursor to move down one line.
There are six control characters that have their own keys (Figure 2-11):
• Rub Out (delete)
• Line Feed (line feed)
• Back Space (backspace)
• Rtn (carriage return)
• Esc (escape)
• Tab (horizontal tab)
Esc
Tab
Ctrl Shift
Back Space
The Ctrl and Shift keys are included in the following descrip-tion of the control keys because they are used to specify control characters that are not represented on the keyboard.
The Rub Out Key. The Rub Out key sends the rub out (or delete) character.
Rub Out is the factory default character delete key in Setup mode. See the explanation of the EDITCHARS command in Section 6.
The Line Feed Key. This key sends the line feed character, which causes the cursor to move down to the next line with-out returning it to the left margin. If the cursor is at the bot-tom of the screen and is not in the dialog area, a line feed moves it to the top of the screen one margin to the right of the current margin. When the cursor is at the bottom of the last column, a line feed either causes it to move to the top of the first column or causes a page-full condition (see PAGEFULL in Section 6).
Rub Out
Return
Line Feed
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Figure 2-11. The Control Keys.
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REV, JAN 1984 ·41158 OPERATORSYou can use the setup command LFCR to cause a line feed received from the host computer (or from the terminal when it is in Local mode) to cause the terminal to generate a line feed and carriage return (see Section 6).
The Back Space Key. The Bacl< Space key sends the back-space character, which moves the cursor one character position to the left.
The following explanation applies when the terminal is not in Setup mode or connected to a host. If the cursor is in the dialog area and is already at the left margin when you press the Back Space key, the cursor does not move. If the cursor is not in the dialog area and is already at the left margin when you press the Back Space key, there is no effect.
The Rtn Key. The Rtn key sends the carriage return control character, which returns the cursor to the left margin of the current line.
Some host computers automatically include a line feed when they echo or send the return character, so that pres-sing the Rtn key moves the cursor down one line and to the left margin. If your host does not, you can use the setup command CRLF (explained in Section 6) to cause the termi-nal to include a line feed with the return character.
In Setup mode, the terminal automatically provides a car-riage return and line feed when you press the Rtn key.
The Esc Key. When you press Esc, the terminal sends the escape character when it is communicating with a host computer.
The escape is the first character in a special command sequence called an escape sequence. Appendix C lists the escape sequence commands. For detailed explanations of all escape sequence commands, see the 4110 Series Host Programmers Reference Manual.
The Tab Key. The Tab key transmits the horizontal tab character.
Depending on how your host treats this character, it can cause the cursor to move a certain number of spaces to the right.
When the terminal is in Local mode, the cursor moves one character to the right each time you press the Tab key.
The Ctrl Key. Some control characters are used often and have their own keys, such as Rtn and Line Feed. Others, however, do not. You can transmit any control character by holding down the Ctrlkey while you press another key.
For example, press Ctrl and the G key (Ctrl-G) at the same time to send the ASCII Bel character, which causes the terminal bell to sound; Ctrl-M sends a carriage return char-acter, causing the cursor to return to the left margin; Ctrl-J sends the line feed character, causing the cursor to go down one line.
The Shift Key. Most keys have two meanings: an unshifted meaning and a shifted meaning.
When you press the key labeled W while holding down the Shift key, the terminal sends the uppercase character W;
pressing the same key without the Shift (if the Caps Lock key light is not on) sends the lowercase letter w. Some keys are labeled with two non-alphabetic characters, one above the other. For example, press the key labeled 5 to transmit a 5; press the same key and the Shift key to send the percent (%) character.
Programmable function keys and the non-programmable command keys can also have shifted and unshifted mean-ings, as explained under the headings Command Keys later in this section.
The Shift key is also used at the same time as the Ctrl key to send some control characters.
The Break ~(ey. The Break key (Figure 2-12) can be used to interrupt communications from some host computers to the terminal. It does not actually send a character to the host;
instead it alters the state of the communications line so that the computer stops sending information to the terminal or stops the execution of a program.
~
Before using the Break key, or the BREAKTIME command, find out how the host computer handles the interrupt. An unexpected Break signal could cause program failure or equipment damage.
4115B OPERATORS REV, JAN 1984 2-13
CONTROLS, INDICATORS, AND KEYBOARD
The length of time that the interrupt lasts is important to some host computers. You can use the setup command BREAKTIME to specify the length of the interrupt sent by the terminal to the host, or to disable the Break key, as explained in Section 5.
It is also important to know that various host computers react differently when they receive the Break. One host might stop the execution of a program while another might log you off and break the communications link to the termi-nal. Therefore, before using the Break key or BREAKTIME command, you should find out how the interrupt is handled by your host.
Pressing the Break key also unlocks the keyboard (see Kybd Lock earlier in this section).
COMMAND KEYS
There are six command keys (Figure 2-13) that perform special functions or put the terminal into a particular mode of operation. These keys cannot be programmed to do any other function. Their purpose is to allow you to easily access frequently used terminal features.
The eight command keys are:
• Page
• Caps Lock
• SETUP
• DIALOG
• CLEAR
• LOCAL
• CANCEL
• COpy Page
When you press the Page key, the following items are erased:
• Graphics in the current view that are not part of a graph-ics segment (a graphics segment is a picture or part of a picture that can be manipulated by terminal commands).
• Text that is not part of a graphics segment and not in the dialog area.
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Figure 2-12. The Break Key.
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REV, JAN 1984 41158 OPERATORSThe following are erased momentarily and then redrawn when you press the Page key:
o Segments visible in the current view.
o The framing box.
o Border around a viewport.
A segment is permanently erased by being deleted from the terminal's memory. A segment can also be made invisible (not erased from the terminal's memory, but not visible on the screen). After a segment has been deleted, you may still need to press Page to erase its image from the screen, depending upon how the terminal's FIXUP parameter is set (see Section 6).
If the dialog area is not enabled, the following actions occur when you press Page:
o The cursor returns to the home position (the upper-left corner of the screen).
o The terminal goes into alpha mode (that is, it interprets all incoming data as alphanumeric data, not graphic data).