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

192 Chapter9
Describing Asynchronous Devices
How to Control Terminals
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.
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.