Radio Shack Quick Printer II Technical Reference Manual

RS232C
INTERFACE
KEYBOARD
INTERFACE
The
RS232C
Interface
utilizes
a
4-pin
DIN
connector
(P2).
This
interface
allows
the
computer
to
have
serial
communica-
tion
with
printers,
modems,
or
other
computers.
The four
signals
used
by
the
interface
are:
1.
CD
-
a
status
input
line
2. RS232IN
—
serial
data
input
3.
GROUND
-
zero
voltage
reference
4.
RS2320UT
-
serial
data
out
The
pinout for
the
DIN
connector
is shown
in
Figure
13.
RS232
OUT
The
components
which
make up the
Keyboard
Interface
are
shown
on sheet
3
of
the
schematic,
upper
left
corner.
The
PIA
chip
(U8)
is the only
digital
circuit
used.
The PIA
chip
is a
programmable
interface
device
which
functions
as both
an
input
and
an output
register.
The eight
keyboard
columns
are
attached
to the
B
side of
the
PIA.
These
eight
lines are
programmed
to be
outputs.
The
seven
keyboard
rows are
attached
to
the
A side of
the PIA.
These
seven PIA
lines
are
programmed
to be
inputs.
To
read
the
keyboard,
only one
column
is enabled by
writing
a
zero
in the
bit that
corresponds
to that
column
and by
writing
ones
in
all the
other
bits. If
a
key
has
been
pressed
in that
column,
one of
the
input
lines
will be
a zero and
the key
location
will
correspond
to
the bit that
is low.
By
scanning
each
column in
the
keyboard,
all
of
the
keys
may
be checked.
SYSTEM
POWER
SUPPLY
FIGURE
13. RS-232C
CONNECTOR
PIN
OUT
In
general,
an
RS232C
signal
is defined
as
a high (or
logic
1
)
if
the
voltage
is greater
than
+3
volts.
It
is
defined
as
a
low
(or
logic
0)
if
the voltage
is less
than
-3
volts.
The
range of
-3
volts
to +3
volts
is undefined.
The
RS232C
interface
circuitry
is
shown
on
sheet 2 of
the
schematic,
upper
right
corner.
For
the
signal
RS2320UT,
an
output
pin of
PIA
U4
(pin
3)
is tied
to
a
741 C Op-amp
(U15).
The
Op-amp
is referenced
at
1.4
volts by
resistors
R23 and
R24.
This
reference
causes
the
Op-amp
to swing
between
the
two
power
supply
voltages
(+/-12
volts)
as the
TTL input
switches
states.
A
100 ohm
resistor
(R27)
is included
to
provide
a current
limit
on
the
output.
The
two
input signals
(RS232IN
and
CD) utilize
identical
circuits.
RS232IN
(pin
2,
U14)
is
tied to
input
pin
10 of
PIA
U4,
and
CD
(pin
1,
U14)
is
tied to
pin interrupt
input
40 of
PIA
U4. The
inputs
from
the
external
device are
connected
to
the
positive
side
of a diode.
This
diode
blocks the
application
of
a
negative
voltage
to the
comparator
(U
14).
When
a
positive
voltage
is applied,
the
diode
conducts,
and the
voltage
is
applied
to
the
input.
The
comparator
is referenced
at
2
volts,
thus if
the
input
voltage
is greater
than 2.6
volts,
the
comparator
is turned
on.
The
comparator
output
is open
-collector
so a
pullup
resistor
is
required
for
the
output.
Also
a
feedback
resistor
of
10M is
used
to prevent
oscillation
or
ringing.
The
Color
Computer
requires
four
supply
voltages: +12
volts,
-12
volts, -5
volts,
and +5
volts.
The
largest
current
requirement
is
on the
+5
volt
supply,
therefore
it
is designed
to provide
up to
1.35
amp.
The
next
largest
requirement
is
400
milliamps
on the
+12
volt
supply.
The
requirements
for
-12
and -5
volts
are very
minimal,
so
the
-5
volt
supply
is
rated
at
100 microamps
and the
-12
volt
supply
at
100
milliamps.
The power
supply
is
shown
on
sheet
3
of
the
schematic
diagram.
On
the
primary
side of
the
transformer,
the
power
supply
incorporates
a
0.7 amp
fuse
and three
high
voltage
capacitors.
These
capacitors
(C20,
C21, and
C22)
provide
noise
immunity
and isolate
computer
noise.
The two
active
lines
of the
power
cord are
connected
across
the primary
side
of
the
power
transformer.
The
third
line
(ground)
is
connected
to the
computer
ground.
The
transformer
has
two
center-tapped
secondary
windings.
One
secondary
is
rated
at
16.3 volts
AC
at
1
amp, and
is used
solely for
the
5
volt
supply.
The other
secondary
is rated
at
33.5 volts
AC at
0.35 amps.
Both
center
taps are
connected
to
ground.
Each
side of
the
16.3
volt
AC secondary
is connected
to
a rectifier
diode
(CR12
and
CR13).
This
produces
a
full-wave
rectified
signal
when the
two
negative
sides
of
the
diodes
are
tied
together.
All
of
the
above
circuitry
is located
on a secondary
board
inside
of the
Computer
case.
This
isolates the
high voltage
AC
signals
away
from
the
main
circuit
board. This
transformer
board
is indicated
on the
schematic
by
a
dashed line.
33

Radio Shack Quick Printer II Specifications

General

Brand	Radio Shack
Model	Quick Printer II
Category	Desktop
Language	English