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13220
Processor Module
13220-91087/18
Rev
JAN-04-B2
As the character
code and
scan line
count is issued to the character
ROM
an
access
tiMe
delay
is
encountered before
the
dot
data
is
available at the outputs.
"he character
ROM
has an access tiMe of 300
ns, therefore one full character tiMe (349 ns) delay is introduced.
As
the
dot data beCOMes available out of
the character
ROM
the
lVSRX
(load video shift register, buffered)
signal is brought low which,
on
the rising edge of
DRCX,
parallel loads the data frOM the character
ROM
into the character shift registers U312 and U313 (and U314 as explained
later).
Since only seven dots per scan line are required for standard
characters,
seven dots are
loaded frOM the character
ROM
(low output
Means dot is lit) into the shift registers.
The MSB
(Most significant
bit)
output frOM the character ROM is latched by U1a
(on LCGAX clock)
and is used to enable the half-shift function
(described below).
The
MSB output of U312 is connected to the serial input of U313 essentially
forMing an 8 bit shift register.
At the saMe tiMe that the seven dots
are loaded into the shift register a 1 is loaded into the MSB.
The QD output of U313 goes to
the character Multiplexor,
U213.
This
Multiplexor
selects
one of several inputs to gate to the
dot streaM.
For a norMal scan
(not half-shift)
the Multiplexor select inputs will
be 101
(C
input is Most significant)
selecting the DS input.
As the
dots are loaded into the shift register the first dot
(which
is high)
appears on the DS
input of
the Multiplexor
and is gated
to the
dot
streaM.
On
each of the next 8 dot
rate clocks
(DRCX)
dot
data is
shifted one bit position in the shift register and therefore to the
DS
Multiplexor input and to the dot
streaM.
Since
the serial
input of
U312 is tied
~igh,
a 1 (blank dot)
is shifted into the shift register
as the dot
data is
shifted
out.
Therefore
at the end of the 9 dot
clocks COMprising the horizontal
scan
for a standard
character,
the
first
and
last dots
are
blanked
(l's)
with
the 7
dots frOM
the
character ROM in between.
HALF-SHIFf
To avoid the "stairstep"
appearance of characters with long diagonals,
a feature known as
"half-shift"
is iMpleMented which allows
and scan
line of a
character to
be
delayed
by
half a dot tiMe.
This half-
shifted scan line,
placed between two norMal scan lines,
fills in the
diagonal as shown below.
X
X
x
X
x
X
(no half-shift>
norMal
half-shifted
norMal
half-shifted
norMal
half-shifted
x
X
X
X
X
X
(with half-shift>
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