Reading
the
Display RAM.
As
we said before,
the
personality card cannot
read
what
is
written
into Display
RAM.
Whatdoes
read
Display
RAM
is
the Character
PROM.
Of
course,
this
isn’t
exactly
true
—
PROM
certainly
can’t
read
RAM.
But
the
result
is
about
the same.
The
Display
RAM
contents
are fed
to
the address
inputs
of
the
Character
PROM,
where
they
are
converted
into
information
suitable
for
7-segment
LEDs.
There
are
eight
7-segment
LED's
in
the
MicroLab! display.
Since the
Character
PROM
can
only
put information on
its
output
lines
for
one
character
at
a
time,
another
device
must turn
on
the
anode
of
each
LED
in
the
proper
sequence.
That
device
is
the Character
Counter,
U3010.
Let's
look
at
how
the
5-bit
segment
word
gets
to
the
LEDs.
Refer
to
Fig.
5-10
and schematics 4 and
6.
©
Ea
Once data
is
stored
in
the
Display
RAM,
the
MicroLab
|
display
LEDs
are automatically
selected and refreshed
by
the Character
Counter.
Character Counter
The
Character
Counter
is a
simple binary
up-counter
whose
outputs
are
clocked from
a
0000"
state
through
to
a
“1111” state
by SCLK.
The
outputs
of
U3010
are
the
synchronizing
elements
in
the
Display logic.
Notice
that
output
pin
13
(SO)
is
connected
to
the
RA
(read
address
A)
inputs
of
both U5020
and
U4020.
SO
is
also
connected to
the
RB
(read
address
B)
inputs
of
U6020
and
U3020. The
S1
output
of
U3010
is
connected
to
the
RB
inputs
of
U5020
and
U4020,
and the
RA
inputs
of
U6020
and
U3020.
In
other
words,
SO
and
S1
provide
the read
addresses
for
the
Display
RAM
RX
RX
Enable
Theory
of
Operation—MicroLab
|
Instruction
Display
RAM.
The
Display
RAM
is
read-enabled
by
a
third
output
of
U3010
called
S2.
So
what
we
have
at
this
point
is
a RAM
address
on
SO
and
S1,
and
a
read-enable
signal
from
S2. When
S2
is
high,
only
devices
U5020
and
U4020
will
place
data
on
the
AO-A4
lines
to
the Character
PROM
When
82
is
low,
only U6020
and
U3020
feed
addresses
to
the
Character
PROM.
Let's
assume
that
SO,
S1,
and
S2
are
all
high. When S2
goes
high, U5020
and
U4020
place
data on their output
pins. That
data
is
fed over
AO-A4 to
the
Character
PROM.
The
Character
PROM
converts the
RAM
output
to 7-
segment
display
code. The
code
is
fed
through
a biasing
resistor
network to
transistor
pack
U4010.
U4010 drives
the
display
LED
anodes
low
in
the
proper
fashion
tocreatea
displayable
character.
But
which
LED
is
selected? That's
taken
care
of
by
the Character
Counter.
Notice
that the
SO,
$1,
and S2
outputs
of
the
counter
are
fed
through P3010
to
the keypad/display
board on
schematic
6.
Scan Decoder
>
4
The
Scan
Decoder (on
schematic
6)
is
a
demultiplexer/decoder
that
decodes
a
binary
input
to
force
one
of
eight
outputs
low.
Each
output
of
the Scan Decoder
is
connected
to
the
cathode
of
an
LED.
So
we
have
the
segment
pattern
coming
in
at
the
upper
right
of
schematic
6,
and
the
LED
selection
lines
coming
in
from
the
left
side
of
the
schematic.
In
this particular
situation, SO-S2
high
forces
pin
4
of
U3060
low,
turning
on DS1081.
Character
Segment
Display
Prom
|
>
Driver
LEDs
Character
Scan
Decoder
Selectors
Fig.
5-10.
MicroLab
|
Display
Block
Diagram.
@
Summary of Contents for 067-0892-00
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