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DIDS402-2AM13
The CTS input pulses used to clock the counter are present once every
13 IJ.s or at a frequency of 76.8 kHz.
These pulses are applied to the first binary
counter, which performs a divide-by-eight operation.
The 9. 6-kHz output pulses are applied to the second binary counter and a
second divide-by-eight operation occurs, thus producing the required l200-baud
pulses.
The l200-baud output is assigned the designation of phase A (0A) and is
used to gate serial data into buffer register Dl (receive) or out of D3 (transmit).
The same 0A pulses are also used as a clock for toggling the la-bit decimal
counter.
This counter is described below.
4-18.9 la-Bit Decimal Counter
The purpose of the la-bit decimal counter is to perform a divide-by-lO
operation to develop start, stop, and parity bits for each transmitted character.
These decoded outputs are also used during receive to determine the position in
time of specific bits of received characters.
For example, the PAR decoded
output is used to determine that the start bit and all seven data bits have been
received.
The parity check, and generation circuitry is then enabled and examined
to determine whether the character received had the correct parity.
4-18.10 l2-Hz Oscillator
The 12-Hz oscillator is a free running multivibrator that produces decoded
outputs of 6 Hz and 12 Hz.
The 6-Hz output is used as a cycle frequency for
cycling various operations performed by display terminal operators (CYCLE
key depressed).
The l2-Hz output is used to repeat the step right ( __ ) or step
left (-) function when these keys are depressed to their second detent position.
4- 19 RAS TER GENERATION
The circuits re sponsible for generating the raste r are shown in figure 4-22.
The raster is produced by exciting the CRT screen phosphor material with an
electron beam.
This occurs only during specific periods of time and results
in the formation of 13 lines capable of containing up to 80 displayable characters
each.
.
The electron beam used for forming the raster is developed by placing
a high potential on the CRT anode element and a relatively low potential on the
CRT cathode element.
The high difference in potential causes electrons to leave
the cathode and bombard the screen phosphor material.
When the phosphor is
thus exited, light is emitted to produce a visible effect.
With no deflection, this
light (or raster) would appear in the center of the screen as a small dot and
would be useless.
To produce a usable raster, the electron beam is moved about
the screen by inducing deflection voltages into the yoke assembly on the CRT
neck.
Since the yoke is placed between the cathode and anode elements, the
beam is deflected electromagnetically before it strikes the screen.
By certain
combinations of varying horizontal and vertical deflection voltages, the beam is
swept across the screen from left to right and downwards from top to bottom.
4-48
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