Configuring Systems for Terminals, Printers, and Other Serial Devices (32022-90051)

Chapter 9 191
Describing Asynchronous Devices
How to Control 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.
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
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 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.