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3.2
VIDEO CIRCUITS
The
video
circuits
can
be
divided
into
four
parts:
CRT
controLLers;
cLock, characters cLock, and
dot seriaLizer; char-
acter
ROM;
and
attribute synchronization and
generation.
For
detaiLed video timing information see Figures 3 and 4.
3.2.1
CRT Controllers (78, 88)
Two InteL 8276 CRT controLLers are used to generate the scan Line
count,
the character count,
the horizontaL and verticaL
timing
signaLs,
and
the
visuaL attributes.
The 8276 CRT
controLLer
contains
two 80-character on-chip row buffers:
one is used as a
circuLating row buffer during each scan Line dispLay;
the
other
is
used
to store prefetched
characters for the
next
dispLay
row.
The
RD
input signaL is never used
by the 8039 processor and
is
permanentLy tied to Logic high.
The
C7P
input signaL when used in conjunction with
CS
whether
the
input information on DBO-DB7 is a command
byte.
indicates
or
data
The
data Lines DBO-DB7 are Linked to the 8039 processor bus
for
transferring
program parameters and dispLay characters from
tWe
processor
to the two CRT controLLers.
The
CS
signaL
is asserted when the 8276s are seLected
by
the
processor
for
setting timing parameters.
It is aLso
used
to
reset the J-K fLipfLop (SE) after each character row refresh.
BRDY
is an interrupt request signaL asserted by the 8276 (7B)
at
the
beginning of each row refresh.
The processor
responds
to
BRDY
by
initiating a DMA process which transfers a bLock of
80
characters from the dispLay RAM to
the"row buffers inside of the
two 8276s.
The
BRDY
signaL, when asserted, causes the foLLowing
CPU read operations to become write operations to the 8276s.
The
Wi
signal is derived from two
signaL
sources:
the
write
signaL
from the CPU when the screen is not being
refreshed,
or
the
read signaL from the CPU when the screen is being refreshed.
It
is a controL signaL used either to strobe data into
the
row
buffers or to set the command/data parameters.
The
ASCII codes of the row buffers are outputted on signaL Lines
CCO-CC6. SignaLs
ceo
through
CC6
and the scan Line
count LCO-LC3
index into the character ROM, and convert an
ASCII code into its
corresponding dot pattern to be dispLayed on the screen.
BS
is the buffer seLection signaL.
It enabLes the current write
operation
to
access
the character row
buffers
inside
the
8276s.
GPAO, GPA1, VSP, RVV, and HLGT are the visuaL attributes
vaLid
during
each character dispLay cycLe.
They are
used
for
logic board module 1-8
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