Configuring Systems for Terminals, Printers, and Other Serial Devices (32022-90057)

Describing Asynchronous Devices
How to Control Terminals
Chapter 9
174
How to Control Terminals
DTS helps control the operation of terminals. It automatically handles the input and
output of data to the terminal as explained earlier in this chapter under “How to Control
the Flow of Data.” Alternative methods for controlling terminals are as follows:
• Echoing characters to the terminal screen as they are typed at the keyboard.
• Allowing special characters to be processed as intended. For example,
[Enter]
signals
the end of input data and the backspace character causes an unwanted character to
be deleted from input data. Other examples of special characters include system
break (
[Break]
) and subsystem break (usually
[CTRL]-Y
).
• Modifying the transmission speed through the SPEED or SET commands.
It is possible to programmatically change the method that the DTS uses to control a
terminal. A program can change the following attributes:
• Parity error checking.
• The method of input for a read from character mode to block mode.
• The set of special characters recognized by the DTC.
• The maximum byte count or a maximum time to enter data.
Each of the methods for controlling terminals is explained in the following pages. Refer
to the Asynchronous Serial Communications Programmer’s Reference Manual for more
information on programmatic device control.
Echo
When characters are typed on a keyboard, the terminal user expects each character to
appear on the screen as it is typed. This is referred to as echo. Any of the following
settings determine whether echo occurs:
• Local echo enabled.
• The modem echo is enabled.
• DTC echo is enabled.
• Typeahead echo is enabled.
Only one of these settings should be enabled at any time. Each of these settings are
described below. Hewlett-Packard recommends setting DTC echo. (DTC echo is
automatically enabled for devices configured as terminals.)
Local Echo
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.