Transmission And Reception; On-Line/Off-Line Mode - Data General DASHER D100 User Reference

Chapter 2
PROGRAMMING
From a programmer's point of view, the DASHER
D100/D200 display terminal consists of two independent
I/O devices — a keyboard input device and a display
output device. Thus, on-line communication between them
must be provided by the computer's software. This means
that all displayable information entered at the keyboard
must be echoed to the display by the executing program.
TRANSMISSION AND RECEPTION
D100/D200 terminals transmit and receive information
serially, one character at a time. The transmission rate and
portions of the serial character format are defined by the
setting of switches located in the display unit.
Data transmission rates range from 50 to 19,200 bits per
second. The format of each serial character appears below.
7 data bits
1 or 0 parity bit — switch-selectable
1 or 2 stop bits — switch selectable
If selected, parity can be even, odd, or mark. The number
of stop bits selected normally depends upon the data
transmission rate. For more information, refer to Chapter
4, Installation.
Once the terminal's transmission rate and character code
' format are selected, all data transmitted by the terminal
will conform to these conventions. Furthermore, all data
received by the terminal must also conform to the selected
conventions; otherwise, a transmission error will occur.
The terminal indicates a transmission error by displaying
a white field that is smaller than the cursor.
~ON-LINE/OFF-LINE MODE
On power-up, the terminal executes a self-test. At the
conclusion of a successful self-test, it senses the
communications interface connector. If the connector is
properly inserted, the terminal enters on-line mode. After
this occurs, the operating mode of the terminal, on- or
off-line, can be selected by the operator at the keyboard.
KEYBOARD
D100/D200 keyboards can transmit the 96 graphic
characters of the ASCII-128 character set as well as 30 of
the 32 ASCII control characters. These codes are listed in
appendices A and B.
The keyboards cannot independently transmit the
following ASCII control characters: Unit Separator (0373)
and Record Separator (0363).
Unit Separator is the header code for a three-code sequence.
The terminal transmits this code sequence in response to a
display command requesting cursor position information.
This is explained more fully under the Read Cursor
Address command description later in this chapter.
User Function Codes
Record Separator is the header code for all user function
codes; i.e., the keyboard transmits this control code as the
first code in each user-function, two-code sequence. The
second code is one of the ASCII characters listed in
Appendix C. D200 keyboards can transmit 75 unique, user
function, two-code sequences while D100 keyboards can
transmit 35 of them. Figures 2.1 and 2.2 define the location
of the user function keys on the D100 and D200 keyboards,