Digilent Pegasus Board - Manual (Page 7)

Pegasus Reference Manual Digilent, Inc. ™
www.digilentinc.com Page 7
The electrostatic force imposed by the grid
pulls away rays of energized electrons as
current flows into the cathodes. These cathode
rays are initially accelerated towards the grid,
but they soon fall under the influence of the
much larger electrostatic force that results from
the entire phosphor-coated display surface of
the CRT being charged to 20kV (or more). The
particle rays are focused to a fine beam as
they pass through the center of the grids, and
then they accelerate to impact on the
phosphor-coated display surface. The
phosphor surface glows brightly at the impact
point, and the phosphor continues to glow for
several hundred microseconds after the beam
is removed. The larger the current fed into the
cathode, the brighter the phosphor will glow.
Between the grid and the display surface, the
beam passes through the neck of the CRT
where two coils of wire produce orthogonal
electromagnetic fields. Because cathode rays
are composed of charged particles (electrons),
they can be deflected by these magnetic fields.
Current waveforms are passed through the
coils to produce magnetic fields that interact
with the cathode rays and cause them to
transverse the display surface in a “raster”
pattern, horizontally from left to right and
vertically from top to bottom.
As the cathode ray moves over the surface of the display, the current sent to the electron guns
can be increased or decreased to change the
brightness of the display at the cathode ray
impact point. Information is only displayed when
the beam is moving in the “forward” direction (left
to right and top to bottom), and not during the
time the beam is reset back to the top left edge
of the display. Much of the potential display time
is therefore lost in “blanking” periods when the
beam is reset and stabilized to begin a new
horizontal or vertical display pass.
The size of the beams, the frequency at which
the beam can be traced across the display, and
the frequency at which the electron beam can be
modulated determine the display resolution.
Modern VGA displays can accommodate
different resolutions, and a VGA controller circuit
dictates the resolution by producing timing
signals to control the raster patterns. The
controller must produce synchronizing pulses at
5V to set the frequency at which current flows
through the deflection coils, and it must ensure
that video data is applied to the electron guns at
the correct time.
Raster video displays define a number of “rows”
that corresponds to the number of horizontal
passes the cathode makes over the display area,
and a number of “columns” that corresponds to
an area on each row that is assigned to one
“picture element” or pixel. Typical displays use
Cathode ray tube display system
Anode (entire screen)
High voltage supply (>20kV) Control board
Deflection coils
Grid
Electron guns
(Red, Blue, Green)
gun
control grid
control
deflection
control
R,G,B signals (to guns)
Sync signals
(to deflection control)
Cathode ray tube
Cathode ray
VGA cable