Heathkit H9 - Page 53

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Page51
@
Pictorial4-3 (Illustration
Booklet,
Page
8) shows
the
block
diagram
of the
character
generator
circuits.
At
the
end
ofvertical retrace,
the
RAM
address
counter
is
initialized
to
the address
for the
upper left
corner
of
the screen.
The
address
counter
addresses
one ofthe
960 RAM
locations.
Each
memory location
contains
the
ASCII
code for
the
character
to be displayed.
The
Character
H
ASCII
code is applied through
a
latch
to the character
generator.
However,
only
the
six
least
significant bits
of
the
7-bitASCII
word areused to
address the charac-
ter
generator.
Sixty-four
characters are stored in the
character
generator
ROM, each at its
own address as
shown in Pictorial
4-4. For instance.
the 6-bit ASCII
word oOto00
addresses the character H.
CHARACTER
As
ADDRESS
Ae
Ar
Ao As
Aa'
o
o
o
o
I
o
I
I
o
I
I
o
o
I
o
I
o
I
I
I
ooo
oo I
'o
I
o
oll
roo
rol
tto
ttl
HHffiffiffiffiHffi
ffiffiHffiffiffiffiffi
Hffiffiffiffiffiffiffi
ffiffiffiHffiffiffiffi
Hffiffiffiffiffiffiffi
ffiHHf,ffiffiffiffi
ffiHffiffiffiffiffiffi
HHHffiffiffiffiffi
ASCIT
BI
T
6 5
4
3 2
1
CHARACTER GENERATOR
INPUT
Ae A6
At Ao As Aa
6-BIT ASCII WORD
o o
I o
o o
PICTORIAL 4.4