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lIunction ...... crlptlon
The display controller outputs data to the display assembly. It performs a complete
display refresh cycle
60 times a second in order to keep the image visible on the display saeen. The display refresh process
is
accomplished using
line-at-a-time scanning.
In
this process, which differs from the dot-at-a-time
raster scan process
used in
CRT monitors, an entire 512-pixel display line is activated at one time.
The GPU controls the storing of data in display memory, and the reading of data for output to the
display assembly. The GPU also controls the timing and sync (synchronization)
circuits,
which output
the sync signals
to
control the display assembly.
The display memory stores data in a bit-mapped format. For each point on the display screen there
is
a corresponding bit in the display memory.
If
the bit
in
the display memory
is
a -I', the corresponding
point on the screen will
be lit;
if the bit
is
a '0', the point will
be
dark.
The GPU performs display memory cycles continuously, but only every other cycle
is
a display refresh
cycle. The GPU uses the alternate memory cycles
to
modify and manage the contents of the display
memory.
The GPU performs the refresh process by reading sequential locations
in
display memory and sending
the information stored in those locations to the display assembly. However, the GPU transfers the
data to the data shifter one 16-bit word at a time. The data shifter consists of two 8-bit shift registers,
one for the odd bits and one for the even
bits
of each word. When these shift registers are fully
loaded, they send the data in serial form two bits at a time (odd and even bits) to the display
assembly.
At the display assembly the incoming bits are entered into two 256-bit shift registers. When the shift
registers are full, the 512 data bits are latched, and the display drivers are activated for the entire line.
Thus, the desired pixels in the selected. line are turned on.
This
process is repeated until each of the
255 lines have been written. The refresh process then begins again at the top of the screen.
Notel
The System
m
operating system provides a display timeout feature. If a unit
is
equipped with System Ill, its display normally times out after 15 minutes of unattended oper-
ation. (The display comes back on when
you
press any key.) You can disable the timeout
feature with the d
i sp_ t i me_o f f
command on the Utilities disc.
The System V operating
system does not implement the display timeout feature.
5.8.2 The
Display A.sembly
The display assembly consists of a 512-pixel by 255-line electroluminescent panel and the associated
driver
circuitry
and mounting hardware. The display assembly
is
a
tuned assembly.
It cannot be
serviced
in
the field. If this assembly
is
faulty, replace it.
5.1 0 Keyboard
The detachable keyboard
is
a compressed ITF (Integrated Terminal Family) keyboard. It contains S'O
keys, including cursor keys, edit keys, a numeric pad, and eight function keys. Autorepeat and n-key
rollover functions are provided.
Localized keyboards are available for several languages. Refer to table 10-1 for a list of these key-
boards and their identification codes.
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