Printed Circuit Board - Data General DASHER D100 User Reference

CRT
Circuit board
Figure 5.1 0 100/D200 Components
PRINTED CIRCUIT BOARD
The circuit board contains four major units: the control
logic, the display logic, the interface logic, and the power
supply. These units, except for the power supply, are
connected by a bi-directional bus as shown in Figure 5.2.
The microprocessor synchronizes the operations of the
other units and coordinates the transfer of information
between them. It also stores the current attributes that
determine how characters will be displayed (e.g., blinking,
underscored, etc.).
The program ROM stores the instructions (firmware) that
the microprocessor executes to control the other units.
The display logic controls cursor positioning, the displaying
of characters, and screen rolling. Its main components are
a display RAM, the CRT control logic, and a character
generator ROM.
The display RAM stores the ASCII codes for the characters
to be displayed on the screen as well as each character's
attributes.
The CRT control logic controls display operations. It is
programmed by the microprocessor to perform the
following functions:
e Initiates screen refreshing by continually providing the
CRT driver circuits with synchronizing signals and the
contents of the display RAM.
e Maintains the cursor address and controls the
positioning of the cursor accordingly.
The character generator ROM stores the bit patterns that
determine how each displayable character appears on the
screen. Different character generator ROMs are used for
the different character fonts.