Heathkit H9 - Page 65

the 8-bit
bus
and
the
direction
of data
flow
is
control-
Ied
by
turning
the appropriate
buffers
and
latches
on
and
off at
the
proper
time
in the
TpU
cycle.
When
data
becomes
available
from
the
UART, parallel
input,
or
keyboard,
the I/O
circuit
board
sends
an
appropriate
data
available
signal
(DAV,
at SZ01-3
or DAVo*n
at
S701-4)
to
logic
array
IC7O2
on
the TpU
circuit
board.
The
logic
array
then
checks
a number
of other
inputs
to
determine
which
buffers
and
latches
on
the RAM
and
counter
circuit
board, parallel
I/O,
serial
I/O,
or
keyboard
circuit
board
will
be turned
on
by TpU
pulse
082
during
the write
cycle.
External
devices
must
be ready
to
accept
new
data,
that
is,
the
UART
transmit-buffer
must
be
empty
(XBEU,
S701-11)
and
the parallel
output
buffer
must
be
empty
(XBEP,
s701-1.2).
The
special
character
inputs
(pins
1,2,
B, and 11
to
logic
array lC7O2)
are
also
checked to
determine
whether
or
not
the
character
on the bus is
a valid
character.
The
special
character
detector
is
on the I/O
circuit board,
and its
outputs
enter
the TPU
circuit
board
as follows:
Control
characters and
rubouts
are not
generally
writ-
ten
into
the RAM unless the
plot
function
is chosen.
However, they are always loaded
into
the UART
and
the
parallel
output
buffers. In the
plot
mode, all
characters
are
written
into
the
RAM. In the normal
mode, the only control
character that can be written
into the RAM is a carriage
retutn, but
only
if
there
is a
space
at the
cursor
location. Flip-flop
lC7O4B
checks
for a space
indication before
the carriage
return is
written.
After all input
conditions
have
settled,
TPU cycle 0B2
enables
the RAM,
parallel,
and
UART outputs
ap-
propriately
at
pins
13
(ANDed
with the 4800 Baud
clock),
14
(load
parallel),
and 1.7
(re-select
RAM) of
lC7O2.
See Pictorial 4-1,5 for a summary of how spe-
cial characters are loaded to and from the bus.
Carriage Return
Space
Rubout
Control Character
Line Feed
Back
Space
Bell
s701,-7
s702-B
s702-3
s702-2
s701-9
s701-8
s703-1,2
CHARACTER
DOES
CHARACTER
LOAD INTO
_
I I
PARALLEL
RAMluARrlour
CURSOR
MOVEMENT
ALL
CONTROL CHARACTERS
-
NORMAL
-
No
Plot
-
Yes
Yes
Yes
Normal
-
None
Plot
-
One space to right
*
Except for:
CARRIAGE RETURN
Normal - Yes
lf
space,
Plot
-
Yes
Yes
Yes
Normal - To
start of
line
Plot
-
one space to right
LINE FEED
Normal
-
No
Plot
-
Yes
Yes
Yes Normal
-
Down
one line
Plot
-
One space to right
BACK
SPACE Normal
-
No
Plot - Yes
Yes Yes
Normal
-
One space to left
Plot - one space to right
BELL
Normal -
No
Plot
-
Yes
Yes
Yes Normal
-None
\
Bell
Plot
- One
space to
right
,
Rings
RUBOUT
Normal
-
No
Plot
- Yes
Yes
Yes
Normal - None
Plot
-
One space ro right
PICTORIAL 4.1,5