SNAX/APN Application Programming Manual

The ITI Interface
SNAX/APN Application Programming Manual—420111-001
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Sending and Receiving Data
Sending and Receiving Data
The ITI protocol enables data to be transmitted one line at a time between an application
process and a terminal. The sending party sends pure text (without control information)
to the receiving party, and SNAX/APN provides screen control.
Each WRITE procedure call issued by the application process is formatted by ITI before
output is presented to the display. ITI inserts a byte (field attribute) into every line
written; each field attribute occupies a character position (blank) on the screen.
Therefore, one byte needs to be reserved for such use. For example, if each line of your
device is 80 characters long, an application can write only 79 single-byte characters to
the screen and have them remain on a single line. Any characters in excess of 79 cause
an overflow to another line, even if the RECSIZE attribute for the LU is specified as 80.
Each READ procedure call issued by the application program takes its input from the
current cursor position on the terminal’s display.
Screen Image
The entire screen is used in a wraparound manner for both input and output. A full line
of equal signs (=) forms a moving screen boundary. This line can also be thought of as
the logical bottom of the screen.
As each new line of text is displayed on the screen, the transient boundary line moves
down one line. When it reaches the bottom of the screen, the transient boundary line
wraps around to the topmost line on the screen.
SCROLL Options of the SCREEN Attribute
The output of a series of WRITE procedure calls (without an intervening READ
procedure call) can generate more lines of text than are physically available on the
screen; for example, when you use FUP to list a large file on your terminal. If this
happens, the displaying of text halts after each screen of text, and the boundary line is
replaced by the message:
When the terminal user presses the ENTER key on the terminal, the boundary line
reappears and ITI sends the next screen of text.
You can, however, eliminate this halt in text display through SCF configuration by
specifying SCROLL in the SCREEN attribute of the ADD LU command. Screens of
text then follow each other without stopping; no messages are displayed.
Note. The documentation for your application should make the terminal user aware that, with
3270-type terminals, the prompt is considered part of any command. If the terminal user
backspaces over the prompt and enters a command, that command fails.
*** press ENTER to continue