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

Describing Asynchronous Devices
How to Control Terminals
Chapter 9
175
Modem Echo
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
DTC echo is enabled for both terminal type file 10 and terminal type fil e18. 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 Echo
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.
Disabling Echo
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.
Special Characters
Special characters are ASCII characters that have certain functions assigned to them;
they signal the DTC to take a particular action. The results of using special characters
are called special control functions. These include the characters described below, as well
as the characters used for protocols. The following characters described here are defaults
and are modifiable by applications.
Backspace
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.
Line Delete
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.
End-of-Record Character
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.