Radio Shack Quick Printer II Technical Reference Manual

CASSETTE
INTERFACE
The
cassette
interface is
composed
of an
output
attenuator
connected
to
the
6-bit D/A,
a
motor
control
circuit,
and
an
input
zero crossing
detector.
However,
most
of the
important
cassette
parameters
are
controlled
by
software.
The
cassette
format
chosen
uses a
sinewave
of
2400
or
1200
Hertz to yield
a
Baud
rate
of
approximately 1500
Baud. In
this
format, a
(or
logic
low) is
represented
by one
cycle
of
1200
Hertz,
and
a
1
(or
logic high)
is
represented
by
one cycle
of 2400
Hertz. A
sample
of data is
shown
in
Figure 12. A
typical
program
tape
would
consist
of
a
leader
of alternating
1's
and 0's,
followed by
one
or more
blocks
of
data.
A
block
of data is
composed
of to
255
bytes
of data
with a
checksum, sync
byte, and
the
block
length.
The
output circuit
utilizes
the
6-bit D/A
to
generate a
sinewave
of 1200
or 2400
Baud. This
signal is
then
attenuated
to
approximately
1 volt and
connected
to the
auxiliary
input
of the
cassette
recorder.
REFERENCE
SQUARE
WAVE
COMPUTER
MEMORY
DATA
D/A
OUTPUT
INPUT TO TAPE
OUTPUT
FROM TAPE
T
. . .
|
|
I .
I
,
I
DATA STORED
»
I
'
I
'
I
"
I
'
I
'
I
»
I '
I
|
N MEMORY
This is shown
inverted to
indicate
possible phase
inversion by
the tape
recorder
FIGURE
12.
SAMPLE
DATA
OF
CASSETTE
FORMAT
The
motor
control
circuit is
controlled
by
an
output
PIA pin.
This
signal is
used
to
switch
transistor
Q4. Q4
provides
sufficient
current
capability
to
drive the
relay
coil.
Diode CR7
is
used to
shunt
voltage surges
whenever
Q4 is
switched.
Diodes
CR9and CR10
perform a
similar
function
for the
relay
switch
contacts.
Whenever
the relay
contacts
are
opened or
closed, a
high
voltage spike
could
occur.
However,
diodes CR9
and CR10
will
shunt away
any
high
voltage.
C80
is
provided to
prevent
RF
noise
associated
with
contact
switching.
The input
circuit is a
zero crossing
detector. R33
is a
termination
resistor
for the
cassette
output.
Resistors
R37
and
R38
are used
to bias
one
input of the
comparator at 1
volt.
The
other
input is also
biased
at
1 volt by R36
and the
series
combination
of R35
and R34.
If the
AC
input
from the
recorder goes
negative,
diode CR4
turns on
and
sets
the input
to the
comparator
equal to
1/2 volt.
Since the
other
input is
biased at 1 volt,
the
comparator
output is
switched to the
high
state.
If the AC
input
from the
recorder
is positive,
diode CR4
is
turned off and
the
input to
the
comparator will be
at some
point
greater than
1
volt, in
which
case, the
comparator
output will be low.
The
comparator
output is
open-collector,
so
pull-up
resistor
R40
is
provided
to
generate a
TTL
signal.
R39
is
used to
prevent
oscillation
of the
comparator.
The final
portion
of
the
cassette circuit
is
capacitor C81
,
which
is
used to
isolate
RF
noise
from
the
cassette cable.
DETAILED
TAPE
FORMAT
INFORMATION
The
standard
TRS-80
Color
Computer
tape is
composed
of the
following
items:
1
.
A
leader
consisting
of 128
bytes
of
Hex 55
2. A
Namefile
block
3. A
blank
section
of tape
approximately
equal to 0.5
seconds
in
length;
this
allows
BASIC
time
to
evaluate
the
Namefile.
4. A
second
leader
of 128
bytes
of Hex 55
5.
One
or
more Data
blocks
6.
An End
of
File block
The block
format
for
Data blocks,
Nanrfefile
blocks,
or
an End
of File
block is
as
follows:
1
.
One
leader
byte
-
55H
2.
One
sync
byte
-
3CH
3.
One
block
type
byte
-
01 H
=
Data
FFH
=
End of
File
00H
=
Namefile
4.
One
block
length byte
-
00H to
FFH
5. Data
-0to 255
bytes
6.
One
checksum
byte
-
the
sum of all
the data
plus
block
type
and block
length
7.
One
leader byte
—
55H
The
End
of File
block is
a
standard
block
with
a
length
of
and
the block
type
equal to
FFH.
The
Namefile
block is
a
standard
block
with a
length
of 15
bytes
(0FH)
and
the block
type
equals
00H.
The
15
bytes
of
data
provide
information
to BASIC
and are
employed as
described
below:
1
.
Eight bytes
for
the
program name
2.
One
file type
byte
-
00H
=
BASIC 01 H
=
Data 02H
=
Machine
Language
3. One
ASCII
flag byte-
00H
=
Binary FFH
=
ASCII
4.
One Gap
flag
byte
-01H
=
Continuous
FFH
=
Gaps
5. Two
bytes
for the
start
address of a
machine
language
program
6. Two
bytes
for the
load
address
of
a
machine
language
program
32

Radio Shack Quick Printer II Specifications

General

Brand	Radio Shack
Model	Quick Printer II
Category	Desktop
Language	English