Heathkit H9 Manual page 53

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P age5 1
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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 A a'
o
o
o
o
I
o
I
I
o
I
I
o
o
I
o
I
o
I
I
I
ooo
o o I
'o
I o
o l l
roo
ro l
tto
ttl
H Hf f if f iff if fi Hff i
ff iff iH f fi ff if fi ff if fi
H ffif f if fi ff if fif f iff i
ff if fif f iHff if fi ff if fi
Hf f if f if fi ff if fi ff if fi
ffi H Hf , ff if fi ff if fi
ffi H f f if fi ff if fi ff if fi
H HHf f if fi ff if fif f i
ASC IT B I T
6 5 4
3 2
1
CHARACTER GENERATOR INPUT A e
A 6
A t
A o A s
Aa
6- B IT A SCI I WOR D
o o
I
o o o
PICTORIAL 4.4