HP OpenVMS Systems Documentation

This chapter describes the use of the terminal driver (TTDRIVER) andthe LAT port driver (LTDRIVER). The terminal driver supports theasynchronous, serial line multiplexers listed in Table 5-1. Theterminal driver also supports the console terminal. The LAT port driveraccommodates I/O requests from application programs; for example tomake connections to remote devices, such as a printer, on a server (seeSection 5.4.4).

5.1 Supported Terminal Devices

In addition to the multiplexers listed in Table 5-1, the terminaldriver supports serial line interfaces. At least one such interface isalways provided and is used to attach the system console terminal. Thisinterface does not allow the setting of multiple terminal speeds,parity, or any maintenance functions, with the exception of theinterface included with the VAX 8200 processor. The terminal devicessupported by the operating system for this interface are listed inTable 5-1.

The remote command terminal, used by the DCL command SET HOST, alsomakes use of the features listed in Section 5.2.

The terminal driver provides the following features:

• Input processing
  • Command-line editing and command recall
  • Control characters and special keys
  • Input character validation (read verify)
  • American National Standard Institute (ANSI) escape sequence detection
  • Type-ahead feature
  • Specifiable or default input terminators
  • Special operating modes, such as NOECHO and PASTHRU
• Output processing
  • Efficiency
  • Limited full-duplex operation
  • Formatted or unformatted output
• Dialup support
  • Modem control
  • Hangup on logout
  • Preservation of process across hangups
• Miscellaneous
  • Terminal/mailbox interaction
  • Autobaud detection
  • Out-of-band control character handling

The terminal driver defines many terminal characteristics and readfunction modifiers, which provide a wide range of options to anapplication program. These options allow multiple levels of controlover the terminal driver's input process, ranging from the default ofcommand line editing that provides a highly flexible user interface, tothe PASTHRU mode, which inhibits input process interpretation of data.

5.2.1.1 Command-Line Editing and Command Recall

The terminal driver input process defines a bounded set of line editingfunctions. You can access these functions with control keys on allkeyboards, and with some special keys on certain keyboards as well. Youcan move the cursor in single-character increments (left arrow orCtrl/D, right arrow or Ctrl/F) or in multicharacter increments, to thebeginning of the line (Ctrl/H) or end of the line (Ctrl/E). Theterminal driver supports both insert character and overstrike charactermodes. The insert or overstrike mode is the terminal's defaultcharacteristic¹ at the beginning of a read operation, butyou can change it with the toggle insert/overstrike key (Ctrl/A). Youcan delete characters in word increments (Ctrl/J or line feed) andbeginning-of-the-line increments (Ctrl/U).

When you use the terminal driver's editing functions, the followingrestrictions result:

• You cannot move the cursor to a previous line after a line wrap.
• You cannot insert a character if the insertion would force a line wrap or if a tab follows the current cursor position.
• You cannot delete a word at the beginning of a line after a line wrap.
• You cannot assign the line editing function to other keys.

Command recall, initiated by Ctrl/B or the up arrow, returns the lastline enteredto the command-line buffer. At this point, you edit or reenter the lineby pressing the Return key. DCL extends command recall to the last 20commands by using the TRM$M_TM_NORECALL modifier to disable theterminal driver's recall mechanism.

Any control key that is not defined by line editing is ignored. Forapplication programs that require control key input but do not performQIO functions with special read modifiers, the SET TERMINAL/NOLINE_EDITDCL command restores most of the terminal driver behavior describedunder OpenVMS Versions 3.0 through 3.7.

5.2.1.2 Control Characters and Special Keys

A control character is a character that controls action at the terminalrather than passing data to a process. An ASCII control character has acode between 0 and 31, and 127 (hexadecimal 0 through 1F, and 7F); thatis, all normal control characters plus DELETE. (Table C-1 lists thenumeric values for all control characters.)

You enter some control characters at the terminal by simultaneouslypressing the Ctrl key and a character key, such as Ctrl/x.Table 5-2 lists the terminal control characters. You can changecontrol character echo strings (Ctrl/C, Ctrl/Y, Ctrl/O, and Ctrl/Z) ona systemwide basis (refer to the OpenVMS System Management Utilities Reference Manual). You enter specialkeys, such as Return, Line Feed, and Escape, by pressing a single key.

Several of the control characters do not function as described if theDCL command SET TERMINAL/LINE_EDIT is not specified. Refer to theOpenVMS DCL Dictionary for information on line editing function keys and the SETTERMINAL command.

The read verify instructions provided by the terminal driver allowvalidation of data as each character is entered. Invalid characters arenot echoed and terminate the operation. The terminal driver does notsupport full function field processing. Large data entry applicationsshould use one of the DECforms, FMS, or TDMS layered products, whichsupport the entire data entry environment. Section 5.4.1.4 describes thesupported primitives.