Data General DASHER D100 User Reference page 42

When the terminal is powered up, the microprocessor
executes a series of diagnostic programs that test the
operation of the control logic and the presence of the
keyboard. As soon as it completes these tests, the
microprocessor initializes the character attributes to a
default state where rolling and blinking are enabled but
no characters will be blinking, dimmed, underscored, or
in reverse video until these attributes are specified.
Besides initializing the character attributes, the
microprocessor turns off the alpha lock to place the
terminal in lower-case mode. In a D100 terminal, it. also
places the keyboard in cursor lock mode to enable the
cursor pad.
Next, the microprocessor places the terminal on-line if the
communications connector is installed; otherwise, the
terminal stays off-line. In on-line mode the microprocessor
constantly scans the keyboard for pressed keys and checks
the communications interface for ASCII codes received
via the commmunications line. In off-line mode, it only
scans the keyboard.
Scanning Keyboard
The microprocessor scans the keyboard by addressing
consecutive keys. If the addressed key is pressed, the sense
circuitry on the keyboard sends a pulse to the
microprocessor. If the same key (or combination of keys) is
pressed for two consecutive scans of the keyboard, the
microprocessor generates the ASCII code for the key (or
combination) from its address. It then transfers this code
to the communications interface for transmission (on line)
or processes the code internally (off line) as if it was
received from the communications interface.
Communications Interface
The microprocessor sends ASCII codes to the
communications interface (on line) where they are
converted into serial format and transferred to the
communications line.
When the terminal receives an ASCII code via the
communications line, the communications interface
assembles the data into an 8-bit buffer. As soon as the code
is assembled, the interface notifies the microprocessor to
retrieve. the code.
Processing Codes
When the microprocessor receives a code from the
communications interface (on line) or senses a pressed key
(off line), it first determines if the code is acontrol character
or a printing character. If it is a control character (000, to
037g), the microprocessor performs the defined function.
For example, a Start Dim command causes the
microprocessor to change the current attribute settings so
that subsequent printing characters are dimmed. If a
printing character (040g to 1763) is received, the
OPERATION
microprocessor stores the character's ASCII code, along
with a copy of the current attributes, in RAM.
Displaying Characters
The CRT control logic continually addresses sequential
locations in display RAM, accessing character codes and
their attributes. In this way, the logic updates the screen as
the microprocessor changes the contents of the display
patterns used to form each character. These bit patterns,
along with the attribute information, enter the CRT driver
required by the CRT/yoke assembly. At the same time,
by the yoke to position the characters on the screen.