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j.3;;!.;;!.()
Pl' OCE-~S~:j.O('
ModulE'
1:5~!20-91
OB'7 119
Rev
JAN .. -04 .... B2
In
the
~:;tandard
charact(~r
set
the
MSH
output
of the
char'actr~r
ROM
indicates that
a
scan line
is
to
be
halfshifted.
This output is
1
C1
t
c h ed
(b
y
L.CGAX)
in t
0
lJ 18
wh
(~r'
e
it
ish e Id
f'
0
r the
(7
dot~;)
0
f the
character tiMe.
The output of
Uta
is fed to the character Multiplexor
select input
A
which, for half-shifted scan lines, selects the
D4
input
(U213 pin 1S).
The QD output of the dot shift register, U313,
is sent
to the JK flip-flop,
U413,
clocked on the falling edge of DRCX, which
perforMS the half-shift of the dot data.
The output of this flip-flop
goes to the
D4
input
of
the
character Multiplexor.
The half-shift
flip···flop
i~:;
pl'eSE-~t
by l..VSRX at
tl'H~
tit'te
n~~w
dots ar'e loaded :i.nto the
~:;hift
r'(-?~J:i~;;t(~7'~:;.
CDPY BIT
SOMe
alternate
character
sets
such
as line drawing
set
or
large
character set require all nine dots an a scan line to be active.
This
allows for
continuous dots across a character boundary as required for
drawing forMS,
etc.
on the display.
In order to get nine dots out of
eight outputs frOM the
character
ROM,
a copy bit circuit is activated
which copies the
M5B
output into the first two dots while the reMaining
seven
ROM
outputs forM the reMaining seven dots.
The seven least significant
ouputs frOM
the character
ROM
are loaded
into shift registers U312 and U313 as
for
standard
characters.
The
MOst significant output is loaded into both the
A
and
B
inputs of shift
register
U314 at
the saMe
tiMe as the least significant seven
bits.
Thu"::;,.
th€-~
MSB
i<.:;
"cop:i.E-~d"
:i.n
<':;hift reg:i.<.:;ter U3t4.
The l'eMaining dots
are brought
frOM the
QC output of U313 into the serial inputs of U314
thereby forMing a nine bit
shift register with U312,
U313,
and U314.
The
QB output frOM U314 is then
fed to
the D7
and D6 inputs
of the
ch'::H'act0~l'
Mult:i.plf.·~xo7'
wh:i.c:h
(-11'€'~
s(~lE-~ctf.~d
whe-m
thE-~
selfo:'ct inputs
ar'e
11X.
Note that
the select A input
is a don't
care since half-shift
cannot be used in these character sets.
The copy bit circuit
15
activated whenever
the
X'7
output of of the
character latch U310 is active.
ReMeMber that this bit is activated to
select the second character set in
a
4K
character
ROM
or the second and
f
0 lH'
t
h
~:; €-~
t
~.;
i n a
n 0 K
c:
h
cH' act
EH'
R
(J
M .
The f
i
l'
S
t
~~ ;:.~
c:
hal' act
E-H'
0
fan y
0
f
the four posible character sets are reserved for control characters and
therefore
copy bit is deactivated
when these positions are
accessed.
'"his condition is decoded by bits
XS
or
X6
being gated with
X'7
(U34 and
U1.12)
to
€,mablf.~
copy
b:i.t only for the UppE-H'
96
chcH'actel'~:;
of
th(~
Sf.~t.
The result of this decoding is latched in the JK flip-flop, U413, which
allows
for the access tiMe
of the
character ROM.
The
flip-flop is
clocked
by the COMbination of LVSRX and
DRCX
which are gated
together
by
U411.
The output of the copy bit enable
latch
is
then used
to
select the copy
bit shift
register
output
and
gate it
to
the dot
s
t
l'
0~a
M .
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