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13220
Processor Module
13220-91087/14
Rev
JAN-04-82
the
NRAS-MUX-NCAS sequence,
accessing the addressed
byte.
Data
is
available 100 ns froM NCAS, and, since NMUX is high, is placed directly
on the Z80A data bus (U&2 is in transparent Mode), and therefore on the
line
buffer inputs.
As the shift register
output
QD
goes low
the
output
of U77
is
forced high and 1'5
are
shifted through the shift
register cOMpleting the RAM cycle.
As MUX goes high again,
NMUX goes
low causing
the data out frOM the RAM to be latched in the transparent
latch,
U62,
where it is held until the next MeMory access.
As LBCDEL
(delayed line buffer clock)
goes low the data is clocked into the line
buffer
U38.
The CRTC increMents the address and the next load signal
occurs 9 dot tiMes frOM the first,
repeating the DMA
cycle.
In this
way 80 sequential bytes of data
are fetched
frOM
the
RAM and loaded
into the line
buffer during the 80 active video character tiMes of the
display.
Note:
Although the
shift register
load
signal is enabled
four
character tiMes before active video,
the
CRTC holds
the starting
address until active
video
and
then increMents
it during active
video.
In addition,
the data is not clocked into the line buffer
until the line buffer clock transitions low during active video.
On the last scan line of a character row, scan line 14, the CRTC lowers
the LBRE
(line buffer recirculate enable) output,
taking line buffers
U28 and U39 out of the recirculate
Mode
(where the
output is shifted
back
into
the input)
thereby
allowing data to
be clocked
into the
inputs.
During the DMA cycle of scan line 14,
as characters are being
output frOM line buffer U39 to the display, characters for the next row
are fetched frOM MeMory and loaded into
line buffer U39.
At the saMe
tiMe,
as enhanceMent data is shifted out frOM U28,
the data which was
previously
stored in the
teMporary line buffer
U38
(during
the DMA
cycle of scan line 6)
is
shifted into
U28.
In this way the display
data for
the next
row of characters is loaded into the
line
buffers
during the last scan line of the
previous row as it is being displayed
on the screen.
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