Theory
of
Operation—MicroLab
|
Instruction
In
this
way,
a
1200
Hz
tone
presents
a "0"
toU2040
whena
clock
pulse occurs.
1/0
Decoding
@
BEA
The
1/O Decoding
logic
on the main
interconnect
board
allows
the personality
card
to
address
the
keypad,
display,
and
1/O
ports.
In
the
upper
left
corner
of
schematic
4,
U8030
constantly
looks
at
BA1,
BA2, BA3,
and_
the
ONBDIO
lines from
the personality
card.
When
ONBDIO
goes
low,
BA1-BA3
select
one
of
the
Y
outputs
of
U8030.
When
output
Y4
is
low,
the
MODEM
Port
ACIA
is
enabled.
When
Y3
goes
low,
the
Cassette
Port
is
enabled.
And,
when
Y2
goes
low,
the
Terminal
Port
is
selected.
The
remaining
two
outputs
select
the
display
and
keypad, which we'll talk
about
next.
Display
Logic
&
HEE
As
far
as the personality card
is
concerned, the
Display
logic
exists as
an
|/Q
port
that
the
personality card writes
to.
When BA1-BA3
and
ONBDIO
are
all
low,
the
YO
output
of
U8030
goes
low.This
low
signals the
Display
logic
that
the personality card
is
going to
write
a value
into Display
RAM
from
BDO-BD7.
This
value
in
RAM
is
converted
by
the
Character
PROM
into
a
set
of
signals
that turn
on specific
segments
of
a
specific
LED in
the
display.
The
Display
RAM
contains
eight 5-bit
data
words,
one
for
each
display
LED.
Once
a
value
is
loaded
into Display
RAM,
the Character Counter
cycles
through
RAM,
reading
the
values
over
and
over
again
into
the
Character
PROM.
This
results
in
the
display being
refreshed at
a
rate determined
by
the
counter. We'll talk
about each
of
these
elements
next.
Display
RAM
&
gH
The
Display
RAM devices
are actually
4
X
4
registers, and
are
set
up
in
pairs
(U6020/U3020 and U5020/U4020)
to
provide
eight 5-bit
data words to
U5010.
For
example,
U6020
provides
four 4-bit
words
to U5010.
Four more 1-bit
words
are
provided
by
U3020, so
that
U6020 and U3020
together
feed four 5-bit words
to U5010.
The
same thing
happens
with
U5020 and
U4020.
The
data stored
in
Display
RAM
is
sent
to
memory on
BDO-BD4.
In
addition,
BD5
and
BD6
provide
the
addressing
bits.
BD7
acts
as
the switching
bit
to
select either
U6020/U3020, or USO20/U4020.
The
contents
of
BDO-BD7
are
shown
in
Fig.
5-9.
5-14
These
Five
Bits
Determine
The
Character
And
Location
Displayed.
Display
RAM
Address
This
Bit
Selects
Either
—
U6020/U3020
or
U5020/U4020
Fig.
5-9.
The
Display RAM
Bit
Format.
The data byte
written
to
the
display
1/0
location
selects
the
character,
character
position,
RAM
location,
and
RAM
devices.
The
bit
pattern
of
BDO-BD4
is
determined
by
adding the
value
of
the
character
you
want
to
display
to
the value
of
the
display location.
Section
2 of
this
manual
tells
how to
program
the
display.
Now,
let's
discuss
how
BD5-BD7
address
and control
the
Display
RAM.
Display Enable
>
4
The
Display
logic is
accessed
by
an
I/O
read
or
write
toOO
on BA1-BA3.
Let’s
look
at
what happens
when
the
personality card
wants
to
write
to Display
RAM.
Writing
to the
Display RAM.
When
YO
of
U8030
goes
low,
the
very
next
“O
to
1”
transition
of
BE
will
clock
a
high
to
output
pin
9.
Then,
if
BD7
is
low,
two
highs
will
appear
on
the
input
of NAND
gate
U4030D.
The
resulting
low
write-
enables
U6020
and
U3020.
If
BD7 is
high,
the
output
of
U4030C
will
go
low,
enabling U5020 and_U4020.
But
notice
the
set
input
of
U106OB.
If
the
BW/R
line
is
low,
indicating a
read
operation,
pin
9 of
U1060B
is
forced
low,
disabling
the write-enable
inputs to
Display
RAM.
The
Display
RAM
can
never
be
read
by
the
personality card
When
the
BD7
line
enables the
GW
input to
a RAM
pair,
BDS5
and
BD6
select
which
of
the
four 4-bit
registers
in
each
RAM
is
to
be
written
to. Remember
that
U3020 and
U4020
use
only
the
least
significant
bit
of
each
register.
Summary of Contents for 067-0892-00
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