How to Control Terminals

• Local echo enabled.

• The modem echo is enabled.

• DTC echo is enabled.

• Typeahead echo is enabled.

When terminals are installed, local echo is turned off. If local echo is turned on, the terminal will echo each character to the screen as it is typed. Local echo is used with half-duplex communications lines and some statistical multiplexers. Note that block mode applications usually turn local echo on because input characters are sent to the DTC in a block of data, instead of one character at a time.

Modems are able to echo characters as they are typed. This feature should not be used. This feature is usually disabled in the modem's default configuration setting.

DTC echo is enabled for both terminal type file 10 and terminal type file 18. The DTC will echo to the terminal screen each character as it is typed. DTC echo provides a simple form of data protection since data that appears after it is typed has been successfully transmitted to the DTC and back to the terminal screen again. Disable DTC echo for block mode reads; Local echo takes over the function of DTC echo because data is transferred to the DTC in a block of characters, instead of one character at a time.

Typeahead allows the terminal user to continuously enter data without having to wait for the system to process the data and return the MPE prompt (:). Typeahead echo mode determines whether input characters will be echoed to the terminal screen once or twice.

Sometimes characters should not be echoed to the screen, for instance, when entering a password. In this case, the computer disables echo. However, if characters should not be echoed to the screen at other times, the SET ECHO=OFF command can be used. Echo can also be disabled by a program through the use of the FCONTROL or FDEVICECONTROL intrinsic. Note that programs controlling block mode reads must disable DTC echo.

When a backspace character is entered, the DTC deletes the previous character from the input data and echoes the backspace character to the terminal (provided echo is enabled). When the echoed backspace character is received, the cursor on the screen moves back one character and is positioned at the character that was deleted. Use [Backspace] or [CTRL]-H to backspace.

To delete a line of data while in character mode, press [CTRL]-X. After [CTRL]-X is pressed, three exclamation points (!!!), along with a carriage return and line-feed, are displayed, signaling that all read data was deleted.

When a terminal operates in character mode, a special character is set to enable the user to end a read. This is called the End-of-Record (EOR) character. It is also called a record terminator or line terminator. Two types of EOR characters exist: standard EOR character and additional EOR character.

Standard EOR Character

When users finish typing a line and press [Return] (the standard EOR character), data is transmitted. The carriage return character terminates the read but it is not included in the data of the read or counted in the number of actual characters read. When the EOR character is detected, a carriage return is echoed to the screen and the ASC software generates a line feed. This places the cursor at the beginning of the next line. The EOR character can be replaced with another character during transparent editing; this is done through the FCONTROL 41 intrinsic.

Additional EOR Character

Also referred to as an alternate EOR character, this character is included in the data and the actual character count (byte count) of the data. The read does not end normally, but terminates in an error stating that the additional EOR character was encountered. The program that initiated the read must recover from the error by deleting the additional EOR character from the input data, subtracting one from the byte count, and sending a carriage return and line feed to place the cursor in its proper place.

AEOR characters can be defined through the FCONTROL 25 or, 40 intrinsics. Note that when a read terminates, the program must call the FCHECK intrinsic to see why the read terminated. If returns an error code of 31, it means the read ended with the additional EOR character.

Up to 16 AEOR's are available but only through the use of FDEVICECONTROL 192, 66.

When [CTRL]-A is entered on the system console, it signals a [CTRL]-A console command. The computer sends the system console a carriage return and line feed along with an equals sign prompt (=), signaling that it is ready for the command. [CTRL]-A commands are allowed only on the system console. For more information on the system console and its special commands, refer to MPE/iX Managing Peripherals.

	NOTE: [CTRL]-A is not accepted as a special character from a terminal that is connected to a DTC.

Using default operating conditions, the ASC software responds to a system break signal from the terminal and alerts the computer. The system break function interrupts the execution of programs, subsystems, and most MPE/iX commands. Once the program or subsystem is interrupted, it can be aborted by entering the ABORT command or the program or subsystem can be restarted by entering the RESUME command.

To transmit a system break signal, press [Break]. System break can be disabled programmatically. Refer to the MPE/iX Intrinsics Reference Manual for more information on system break.

Some application programs, such as block mode programs, change the settings of terminals and the characteristics of their device files. These programs should return the devices to normal operating mode before they complete.

If a program does not disable break (through the FCONTROL or FDEVICECONTROL intrinsic), the user can still use [Break]. If [Break] is pressed, the program might not be able to recover but the MPE/iX operating system will by assuming that the terminal is in normal operating mode.

A VPLUS block mode application can be reset by pressing [CTRL], [Shift], and [Reset] simultaneously. Other block mode applications are harder to reset. Sometimes, it is necessary to turn the terminal off and on. Other times, the port must be reset. Refer to Troubleshooting Terminal, Printer, and Serial Device Connections for information on resetting ports if host-based management is being used. Refer to Using OpenView DTC Manager for more information on resetting ports if PC-based management is being used.

Subsystem break stops a program-local or subsystem-local command and enables the user to enter a different command. The default subsystem break character in standard editing mode is [CTRL]-Y.

Subsystem break is not enabled by default. It must be enabled programmatically through the FCONTROL 17 intrinsic. A subsystem break character can also be defined when entering transparent editing through the FCONTROL 41 intrinsic.

Characters are transmitted to the DTC as they are typed. The DTC expects to receive them this way. If DTC echo is enabled, characters are echoed back to the screen as they are received.

Character mode is how terminals are opened, whether by a session or a program. Character mode reads are terminated with the carriage return character ([Return] or [CTRL]-M).

When a terminal operates in block mode, characters are held in the terminal's memory (buffer) as they are typed. Characters are not transmitted to the DTC until a specific action is taken. Pressing [Enter] at the end of a block of data will normally send it to the DTC.

An application program can use an alternate method to end a block mode read. Refer to "Terminating Reads" later in this chapter for more information on ways to end a block mode read.

Block mode is enabled programmatically by executing a block mode application at the terminal. The program needs to change the terminal's configuration settings. It should do this automatically by sending escape sequences to the terminal.

Blocks of characters can be input to the DTC a line at a time (called line block mode) or a page at a time (called page block mode), depending on the type of block mode used. Because characters are displayed on the screen as they are entered, local editing and cursor control features can be used to alter a line or page before it is transmitted to the DTC.

Data is transmitted one character (literally one byte) at a time, with each character bound by start bits and stop bits. Block mode sends data in one transmission "burst," although the characters are transmitted one by one in a long stream.

• Standard editing.

• Transparent editing.

• Binary editing.

• An EOR or AEOR character is sent from the terminal.

• The input byte count is reached.

• The read limit timer expires.

• The block mode read timer expires.

• The system break character (if it is enabled) is sent from the terminal.

• The subsystem break character is sent from the terminal.

• The [CTRL]-A character is sent from the system console.

• The DTC encounters a parity error (only when parity checking is enabled).

• The input byte count is reached.

• The read limit timer expires.